A process for producing a composition having a ratio by weight of C10-C26 monobranched fatty acids or alkyl esters thereof to C10-C26 polybranched fatty acids or aikyl esters thereof of greater than 6 using a zeolite, preferably ferrierite, isomerisation catalyst. The zeolite catalyst is preferably the only isomerisation catalyst used. The zeolite catalyst can be reused many times after simple separation from the reaction products without having to be regenerated.