Process for synthesizing a heminitrile of formula CN-(CH 2) n -COOH or of formula CN-R'-COOH, in which formulae n is between 4 and 13 limits included and R' represents an alkylene radical comprising from 4 to 13 carbon atoms and from 0 to 2 double bonds, with said synthesis being carried out from a compound of unsaturated fatty acid, including ester or glyceride, type of natural origin corresponding to the formula (R 1 -CH=CH- [(CH 2) q -CH=CH] m - (CH 2) r -COO-) p-G in which formula: R1 is H, or an alkyl radical having from 1 to 11 carbon atoms comprising, where appropriate, a hydroxyl function, q is an index which is 0 or 1, m and p are whole indices, m being 0, 1 or 2 and p being between 1 and 3 limits included, if p is 1, in this case G is an H, an alkyl radical having from 1 to 11 and preferably from 2 to 11 carbon atoms, or a radical comprising two or three carbon atoms, bearing one or two hydroxyl function (s), if p is 2, in this case G is the residue of a diol or of glycerol bearing a hydroxyl function, if p is 3, in this case G is the residue of glycerol, r is a whole index between 4 and 13 limits included, with the C=C double bonds in said formula, which may be in cis or trans conformation, and with said process, comprising a first step of ammoniation of the compound of unsaturated fatty acid, ester or glyceride type, resulting in the corresponding unsaturated nitrile, which nitrile is subjected, in a second step, to an oxidative cleavage in two successive phases with the formation of intermediate compounds of vicinal diol type, using H 2 O 2 as oxidizing agent, in at least one of the two phases, so as to result in said heminitrile, said process using, as unsaturated fatty acid of natural origin, oleic acid or lesquerolic acid or vaccenic acid or gondoic acid and with as the nitrile used in the second step being oleonitrile, the nitrile of lesquerolic acid or the nitrile of vaccenic acid or the nitrile of gondoic acid. R1 is H, or an alkyl radical having from 1 to 11 carbon atoms comprising, where appropriate, a hydroxyl function, q is an index which is 0 or 1, m and p are whole indices, m being 0, 1 or 2 and p being between 1 and 3 limits included, if p is 1, in this case G is an H, an alkyl radical having from 1 to 11 and preferably from 2 to 11 carbon atoms, or a radical comprising two or three carbon atoms, bearing one or two hydroxyl function (s), if p is 2, in this case G is the residue of a diol or of glycerol bearing a hydroxyl function, if p is 3, in this case G is the residue of glycerol, r is a whole index between 4 and 13 limits included, with the C=C double bonds in said formula, which may be in cis or trans conformation, and with said process, comprising a first step of ammoniation of the compound of unsaturated fatty acid, ester or glyceride type, resulting in the corresponding unsaturated nitrile, which nitrile is subjected, in a second step, to an oxidative cleavage in two successive phases with the formation of intermediate compounds of vicinal diol type, using H 2 O 2 as oxidizing agent, in at least one of the two phases, so as to result in said heminitrile, said process using, as unsaturated fatty acid of natural origin, oleic acid or lesquerolic acid or vaccenic acid or gondoic acid and with as the nitrile used in the second step being oleonitrile, the nitrile of lesquerolic acid or the nitrile of vaccenic acid or the nitrile of gondoic acid.