A process for synthesizing a branched unsaturated fatty compound of formula III: A-CH=CB'-B in which: A is a radical consisting of an linear, optionally unsaturated, alkyl chain, comprising from 1 to 20 carbon atoms and comprising an acid, ester (of a monoalcohol or of a polyol) or nitrile end function, B' is H or an alkyl chain comprising from 1 to 10 carbon atoms, and B is: - if B' is an alkyl chain, B is an alkyl, optionally branched, chain comprising from 1 to 11 carbon atoms, - if B' is H, B is an alkyl chain comprising from 3 to 11 carbon atoms bearing at least one branched alkyl radical of formula C n H 2n+1 with n in the range from 1 to 5, said process comprising metathesis in the presence of a metathesis catalyst between: - on the one hand, a linear unsaturated fatty compound of formula I: R-(CH 2)a-[(CH=CH)-(CH 2)] b -(CH 2) c -R' in which R' is a radical -COOH, -COOR 1 or -CN, R 1 being a monoalcohol or a polyol residue, such as diol or triol, R is H or an acid, monoalcohol ester or nitrile function, and a, b, c are integers such that 0 ≤ a ≤ 11; 1 ≤ b ≤ 6; 2 ≤ c ≤ 12, the sum a + 3*b + c + 1 being in the range from 8 to 36, preferably from 8 to 24, preferably from 10 to 22 and even more preferably from 10 to 18; and - on the other hand, a branched olefin II comprising from 4 to 23 carbon atoms, in which the main chain comprises at least one branching of formula C n H 2n+1 with n in the range from 1 to 5 when B' is H. A is a radical consisting of an linear, optionally unsaturated, alkyl chain, comprising from 1 to 20 carbon atoms and comprising an acid, ester (of a monoalcohol or of a polyol) or nitrile end function, B' is H or an alkyl chain comprising from 1 to 10 carbon atoms, and B is: - if B' is an alkyl chain, B is an alkyl, optionally branched, chain comprising from 1 to 11 carbon atoms, - if B' is H, B is an alkyl chain comprising from 3 to 11 carbon atoms bearing at least one branched alkyl radical of formula C n H 2n+1 with n in the range from 1 to 5, - if B' is an alkyl chain, B is an alkyl, optionally branched, chain comprising from 1 to 11 carbon atoms, - if B' is H, B is an alkyl chain comprising from 3 to 11 carbon atoms bearing at least one branched alkyl radical of formula C n H 2n+1 with n in the range from 1 to 5, - on the one hand, a linear unsaturated fatty compound of formula I: R-(CH 2)a-[(CH=CH)-(CH 2)] b -(CH 2) c -R' in which R' is a radical -COOH, -COOR 1 or -CN, R 1 being a monoalcohol or a polyol residue, such as diol or triol, R is H or an acid, monoalcohol ester or nitrile function, and a, b, c are integers such that 0 ≤ a ≤ 11; 1 ≤ b ≤ 6; 2 ≤ c ≤ 12, the sum a + 3*b + c + 1 being in the range from 8 to 36, preferably from 8 to 24, preferably from 10 to 22 and even more preferably from 10 to 18; and R' is a radical -COOH, -COOR 1 or -CN, R 1 being a monoalcohol or a polyol residue, such as diol or triol, R is H or an acid, monoalcohol ester or nitrile function, and a, b, c are integers such that 0 ≤ a ≤ 11; 1 ≤ b ≤ 6; 2 ≤ c ≤ 12, the sum a + 3*b + c + 1 being in the range from 8 to 36, preferably from 8 to 24, preferably from 10 to 22 and even more preferably from 10 to 18; and - on the other hand, a branched olefin II comprising from 4 to 23 carbon atoms, in which the main chain comprises at least one branching of formula C n H 2n+1 with n in the range from 1 to 5 when B' is H.