A process for preparing 2-methyl-3-(4-tert-butylphenyl)propanal of the formula I with less than 0.3% by weight content of 2-methyl-3-(3-tert-butylphenyl) propanal of the formula II based on the total mass of the compounds of the formulae I and II starting from 4-tert-butyltoluene of the formula III with from 0.5 to 5% by weight content of 3-tert-butyltoluene of the formula IV based on the total mass of the compounds of the formulae III and IV, as starting material, comprising the steps of: a) electrochemical anodic methoxylation of a mixture comprising compounds of the formulae III and IV, and also comprising the benzyl ethers of the formulae V and VI which are produced as intermediates to give the dimethyl acetals of the formulae VII and VIII in an electrolysis solution comprising methanol, at least one conducting salt, and also optionally one cosolvent or a plurality of various cosolvents; b) hydrolysis of the mixture of the dimethyl acetals of the formulae VII and VIII formed in step a) to give the corresponding benzaldehydes of the formulae IX and X and then use of distillation to reduce the concentration of the benzaldehyde of the formula X; where the distillation in step b) is carried out in two stages, by using a 1st distillation column in a first distillation to remove, at the top of the 1st distillation column, a distillate which comprises more than 80% by weight of a mixture composed of unreacted starting materials of the formulae III and IV, and also of the benzyl ethers of the formulae V and VI produced as intermediates in step a), and of the benzaldehyde of the formula X, and using a 2nd distillation column in a second distillation to remove, at the top of the 2nd distillation column, the benzaldehyde of the formula IX with less than 0.5% by weight content of the benzaldehyde of the formula X, based on the total mass of the compounds of the formulae IX and X, from the bottom product of the first distillation, and where optionally the distillate which is obtained at the top of the 1st distillation column and which comprises the compounds of the formulae III, IV, V, VI, and X is reused in step a) together with the 4-tert-butyltoluene of the formula III that is used as starting material, c) reaction of the mixture of the benzaldehydes of the formulae IX and X obtained in step b), where the mixture comprises less than 0.3% by weight of the benzaldehyde of the formula X, with propanal under basic conditions to give the dehydrocinnamaldehydes of the formulae XI and XII; d) catalytic hydrogenation of the dehydrocinnamaldehydes of the formulae XI and XII prepared in step c) to give the propanals of the formulae I and II, where the content of the propanal of the formula II is less than 0.3% by weight, based on the total mass of the compounds of the formulae I and II, optionally with subsequent use of distillation to remove solvent residues, precursors, and byproducts; e) optionally final distillation of the propanals of the formulae I and II obtained in step d), in order to reduce the concentration of the propanal of the formula II. a) electrochemical anodic methoxylation of a mixture comprising compounds of the formulae III and IV, and also comprising the benzyl ethers of the formulae V and VI which are produced as intermediates to give the dimethyl acetals of the formulae VII and VIII in an electrolysis solution comprising methanol, at least one conducting salt, and also optionally one cosolvent or a plurality of various cosolvents; b) hydrolysis of the mixture of the dimethyl acetals of the formulae VII and VIII formed in step a) to give the corresponding benzaldehydes of the formulae IX and X and then use of distillation to reduce the concentration of the benzaldehyde of the formula X; where the distillation in step b) is carried out in two stages, by using a 1st distillation column in a first distillation to remove, at the top of the 1st distillation column, a distillate which comprises more than 80% by weight of a mixture composed of unreacted starting materials of the formulae III and IV, and also of the benzyl ethers of the formulae V and VI produced as intermediates in step a), and of the benzaldehyde of the formula X, and using a 2nd distillation column in a second distillation to remove, at the top of the 2nd distillation column, the benzaldehyde of the formula IX with less than 0.5% by weight content of the benzaldehyde of the formula X, based on the total mass of the compounds of the formulae IX and X, from the bottom product of the first distillation, and where optionally the distillate which is obtained at the top of the 1st distillation column and which comprises the compounds of the formulae III, IV, V, VI, and X is reused in step a) together with the 4-tert-butyltoluene of the formula III that is used as starting material, c) reaction of the mixture of the benzaldehydes of the formulae IX and X obtained in step b), where the mixture comprises less than 0.3% by weight of the benzaldehyde of the formula X, with propanal under basic conditions to give the dehydrocinnamaldehydes of the formulae XI and XII; d) catalytic hydrogenation of the dehydrocinnamaldehydes of the formulae XI and XII prepared in step c) to give the propanals of the formulae I and II, where the content of the propanal of the formula II is less than 0.3% by weight, based on the total mass of the compounds of the formulae I and II, optionally with subsequent use of distillation to remove solvent residues, precursors, and byproducts; e) optionally final distillation of the propanals of the formulae I and II obtained in step d), in order to reduce the concentration of the propanal of the formula II.