Method for preparing isotope labelled alkane sulfonic acid, especially methane sulfonic acid, comprising the following steps: i) providing an initiator precursor, ii) providing SO 3 and iii) providing the alkane, iv) reacting initiator precursor, SO 3 and alkane in a reactor under a pressure of 1 bar up to 200 bar, and thereby controlling the temperature to be within a range of from 0 °C to 100 °C, v) separating the thus obtained labelled alkane sulfonic acid from non-reacted educts and/or where appropriate side products, characterized in that the obtained alkane sulfonic acid is labelled with at least one isotope selected from 2 H, 3 H, 11 C, 13 C, 14 C, 17 O, 18 O, 33 S, 34 S, 35 S, 36 S, 38 S as well of mixtures of two or more of them, wherein the concentration of the isotope is at least twice the amount of the natural amount of the respective isotope, wherein - the hydrogen isotope(s) in the alkane sulfonic acid are provided by using D 2 SO 4 and T 2 SO 4 respectively as solvent of the reaction and/or to provide SO 3 as oleum and/or - the C isotope(s) in the alkane sulfonic acid is provided by using the corresponding C-labelled alkane as educt and/or - the oxygen isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent, and/or - the sulfur isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent. i) providing an initiator precursor, ii) providing SO 3 and iii) providing the alkane, iv) reacting initiator precursor, SO 3 and alkane in a reactor under a pressure of 1 bar up to 200 bar, and thereby controlling the temperature to be within a range of from 0 °C to 100 °C, v) separating the thus obtained labelled alkane sulfonic acid from non-reacted educts and/or where appropriate side products, characterized in that the obtained alkane sulfonic acid is labelled with at least one isotope selected from 2 H, 3 H, 11 C, 13 C, 14 C, 17 O, 18 O, 33 S, 34 S, 35 S, 36 S, 38 S as well of mixtures of two or more of them, wherein the concentration of the isotope is at least twice the amount of the natural amount of the respective isotope, wherein - the hydrogen isotope(s) in the alkane sulfonic acid are provided by using D 2 SO 4 and T 2 SO 4 respectively as solvent of the reaction and/or to provide SO 3 as oleum and/or - the C isotope(s) in the alkane sulfonic acid is provided by using the corresponding C-labelled alkane as educt and/or - the oxygen isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent, and/or - the sulfur isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent. characterized in that the obtained alkane sulfonic acid is labelled with at least one isotope selected from 2 H, 3 H, 11 C, 13 C, 14 C, 17 O, 18 O, 33 S, 34 S, 35 S, 36 S, 38 S as well of mixtures of two or more of them, wherein the concentration of the isotope is at least twice the amount of the natural amount of the respective isotope, wherein - the hydrogen isotope(s) in the alkane sulfonic acid are provided by using D 2 SO 4 and T 2 SO 4 respectively as solvent of the reaction and/or to provide SO 3 as oleum and/or - the C isotope(s) in the alkane sulfonic acid is provided by using the corresponding C-labelled alkane as educt and/or - the oxygen isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent, and/or - the sulfur isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent. - the hydrogen isotope(s) in the alkane sulfonic acid are provided by using D 2 SO 4 and T 2 SO 4 respectively as solvent of the reaction and/or to provide SO 3 as oleum and/or - the C isotope(s) in the alkane sulfonic acid is provided by using the corresponding C-labelled alkane as educt and/or - the oxygen isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent, and/or - the sulfur isotope(s) are provided by using the correspondingly labelled SO 3 as well as the correspondingly labelled H 2 SO 4 as educt and/or solvent.