Process for relaxing the hair comprising: (i) a step of applying to the hair a relaxing composition containing at least one denaturing agent with a molecular mass of greater than 18.1 g/mol, present in a molar concentration of between 1M and 8M, (ii) a step of raising the temperature of the hair, using a heating means, to a temperature of between 110 and 250°C, the denaturing agent being a guanidine of general formula below: in which: R1, R2, R3, R4 and R5 represent, independently: (vii) a hydrogen atom, or (viii) a linear or branched C1-C4 lower alkyl or alkenyl radical, optionally substituted with one or two radicals chosen from: hydroxyl, amino, dimethylamino, methoxy, ethoxy, carboxyl, carboxamide, N-methylcarboxamide or SO 3 H when R1, R2, R3 and R4 represent a hydrogen atom, R5 may also denote a radical chosen from the following: acetyl; chloroacetyl; carboxamide; methoxy; ethoxy; 1,2,4-triazolyl; cyclopentyl; methoxycarbonyl; ethoxycarbonyl; CO-CH=CH-COOH; phenyl optionally substituted with a chlorine atom or a hydroxyl radical; benzyl; thiazolidone; benzimidazole; benzoxazole; benzothiazole; or C(=NH)-NR6R7 in which R6 and R7 denote, independently of each other, a hydrogen atom or a linear or branched C1-C4 lower alkyl radical, optionally substituted with one or 2 radicals chosen from: hydroxyl, amino, dimethylamino, carboxyl and carboxamide; or N-methylcarboxamide; or alternatively a phenyl radical when R1=R2=R3=H, R4 and R5 may also form, with the nitrogen atom that bears them, a pyrrolidine, piperidine, pyrazole or 1,2,4-triazole ring, optionally substituted with 1 or 2 radicals chosen from: hydroxyl, amino and carboxyl when R1=R2=H, and R4=H or methyl, R3 and R5 may also together form a 5-membered ring optionally containing an oxo group and the organic or mineral salts thereof characterized in that the guanidine of formula is the sole relaxing active agent and it is other than tetramethylguanidine. (i) a step of applying to the hair a relaxing composition containing at least one denaturing agent with a molecular mass of greater than 18.1 g/mol, present in a molar concentration of between 1M and 8M, (ii) a step of raising the temperature of the hair, using a heating means, to a temperature of between 110 and 250°C, R1, R2, R3, R4 and R5 represent, independently: (vii) a hydrogen atom, or (viii) a linear or branched C1-C4 lower alkyl or alkenyl radical, optionally substituted with one or two radicals chosen from: hydroxyl, amino, dimethylamino, methoxy, ethoxy, carboxyl, carboxamide, N-methylcarboxamide or SO 3 H (vii) a hydrogen atom, or (viii) a linear or branched C1-C4 lower alkyl or alkenyl radical, optionally substituted with one or two radicals chosen from: hydroxyl, amino, dimethylamino, methoxy, ethoxy, carboxyl, carboxamide, N-methylcarboxamide or SO 3 H when R1, R2, R3 and R4 represent a hydrogen atom, R5 may also denote a radical chosen from the following: acetyl; chloroacetyl; carboxamide; methoxy; ethoxy; 1,2,4-triazolyl; cyclopentyl; methoxycarbonyl; ethoxycarbonyl; CO-CH=CH-COOH; phenyl optionally substituted with a chlorine atom or a hydroxyl radical; benzyl; thiazolidone; benzimidazole; benzoxazole; benzothiazole; or C(=NH)-NR6R7 in which R6 and R7 denote, independently of each other, a hydrogen atom or a linear or branched C1-C4 lower alkyl radical, optionally substituted with one or 2 radicals chosen from: hydroxyl, amino, dimethylamino, carboxyl and carboxamide; or N-methylcarboxamide; or alternatively a phenyl radical when R1=R2=R3=H, R4 and R5 may also form, with the nitrogen atom that bears them, a pyrrolidine, piperidine, pyrazole or 1,2,4-triazole ring, optionally substituted with 1 or 2 radicals chosen from: hydroxyl, amino and carboxyl when R1=R2=H, and R4=H or methyl, R3 and R5 may also together form a 5-membered ring optionally containing an oxo group and the organic or mineral salts thereof characterized in that the guanidine of formula is the sole relaxing active agent and it is other than tetramethylguanidine.