A transesterification process for the preparation of tetrakis[3-(3,5-di-tert-butyl-4-hydroxy phenyl)propionyl oxymethyl]methane by the reaction of methyl-(3,5-di-tert-butyl-4-hydroxy phenyl)propionate ester with pentaerythritol wherein the reaction takes place in the presence of an ester exchange catalyst combination consisting of (a) at least one basic or neutral catalyst and (b) at least one metal compound capable of behaving as a Lewis acid being different from the basic or neutral catalyst and wherein the reaction is conducted through a first stage in which only the at least one basic or neutral catalyst is present in the reaction mixture followed by a second stage which commences with the addition of Lewis acid catalyst to the reaction mixture when the amount of di-substituted intermediate product contained within the reaction mixture has been reduced to less than 20 area % analysed by HPLC.