A process for producing C10-C26 monobranched fatty acids or alkyl esters thereof which includes isomerizing unsaturated C10-C26 fatty acids or alkyl esters thereof in the presence of a catalyst which comprises both a zeolite and a Lewis base. The zeolite can be reused after simple separation from the reaction products without having to regenerate. The process is particularly suitable for producing highly monobranched fatty acids or alkyl esters thereof.