A process for the reactivation of an acidic ion exchange resin is described. The invention relates to the treatment of an at least partially deactivated resin which has been deactivated by contact with an impure ethylenically unsaturated acid or ester containing target impurities. The reactivation includes the step of contacting the at least partially deactivated resin with an alcohol to thereby increase the activity thereof. The invention extends to reactivating a resin deactivated by contact with an impure ethylenically unsaturated acid, ester or nitrile containing target impurities by contacting the at least partially deactivated resin with an alcohol and a carboxylic acid to thereby increase the activity thereof. A reactivated resin and a process for preparing and purifying an ethylenically unsaturated acid or ester of the following formula: —R1—C(═(CH2)m)—COOR2 are also described.