Smoothing method of hair comprising (i) a step of applying to the hair a hair straightening composition containing at least one denaturing agent molecular mass greater than 18.1 g / mol, present at a molar concentration of between 1M and 8M, (ii) a step of raising the temperature of the hair, with the help of a heating medium at a temperature between 110 and 250 ° C, being the denaturant a guanidine of general formula (III): wherein: R1, R2, R3, R4, R5 independently represent a hydrogen atom or an alkyl or lower alkenyl of C1-C4, linear or branched, optionally substituted by one or two radicals selected from: hydroxyl, amino , dimethylamino, methoxy, ethoxy, carboxyl, carboxamide, N-methylcarboxamide or SO3H, where R1, R2 and R3 and R4 represent a hydrogen atom, R5 can also denote an acetyl radical I chloroacetyl carboxamide methoxy ethoxy 1,2,4-triazolyl cyclopentyl methoxycarbonyl ethoxycarbonyl CO-CH> = CHCOOH phenyl optionally substituted by a chlorine atom or a hydroxyl radical benzyl thiazolidone benzimidazole benzoxazole benzothiazole or C (> = NH) -NR6R7 wherein R6 and R7 denote, independently of one another, a hydrogen atom or a lower alkyl radical of C1-C4, linear or branched, optionally substituted by one or two radicals selected from: hydroxyl, amino, dimethylamino, carboxy or carboxamide or N-methylcarboxamide or alternatively a phenyl radical when R1> R2 => = R3> = H, R4 and R5 may also form, with the nitrogen atom to which they are attached, a pyrrolidine cycle, piperidine, pyrazole or 1,2,4-triazole optionally substituted with 1 or 2 radicals selected from: hydroxyl, amino or carboxyl when R1> = R2> = H, and R4> = H or methyl, R3 and R5 may also together form a cycle of 5 members optionally containing an oxo group and organic or mineral salts, characterized in that the guanidine of formula ( III) is the only active hair straightening and which is different from tetramethylguanidine.Procedimiento de alisado de los cabellos que comprende (i) una etapa
