The origin of the contrasting regioselectivities in allylic substitution found for a heterocuprate MeCu(CN)Li and a homocuprate Me2CuLi was studied using density functional calculations. The gamma-selectivity of MeCu(CN)Li is determined at the oxidative addition stage of the reaction, where the different degree of trans effect of the Me and the CN groups dictates the relative orientation of the methyl group and the leaving acetate group. As the result, the transition state where the acetate group leaves trans to the Me group on the copper atom is favored, and the gamma-selectivity results. The homocuprate Me2CuLi is symmetrical by nature and does not show such regioselectivity.