A process for preparing and purifying an ethylenically unsaturated acid or ester of the following formula:- R 1 -C(=(CH 2) m)-COOR 2 wherein R 1 and R 2 are each, independently, hydrogen or an alkyl group with 1 to 12 carbon atoms; and m is 1 or 2; the process comprising the steps of- a) contacting an alkanoic acid or ester of the formula R 2 -CH 2 -COOR 3, wherein R 1 is as defined for the ethylenically unsaturated acid or ester above and R 3 is independently, hydrogen or an alkyl group with 1 to 12 carbon atoms, with a methylene or ethylene source of formula I, optionally in the presence of a suitable catalyst, where R 5 and R 6 are independently selected from C 2 -C 12 hydrocarbons or H; X is either O or S; n is an integer from 1 to 100; and m is 1 or 2; and optionally in the presence of an alkanol to produce an impure ethylenically unsaturated acid or ester of the formula:- R 2 -C(=(CH 2) m)COOR 2; b) purifying the impure product of step a) by contact with an acidic ion exchange resin until the ion exchange resin is at least partially deactivated; and c) treating the at least partially deactivated ion exchange resin with an alkanol of formula R 4 OH, wherein R 4 is an alkyl group with 1 to 12 carbon atoms, and a carboxylic acid of formula:- R 1 -CH 2 -COOH to thereby reactivate the acidic ion exchange resin and produce as a by-product an ester of formula:- R 1 -CH 2 -COOR 4; d) optionally hydrolysing the ester of step c) to produce an acid of formula:- R 1 -CH 2 -COOH; e) recycling the said ester or acid of step c) or acid of step d) as a reactant in step a) with the proviso that when the ester of step c) is recycled, R 4 and R 3 are the same alkyl group. wherein R 1 and R 2 are each, independently, hydrogen or an alkyl group with 1 to 12 carbon atoms; and m is 1 or 2; the process comprising the steps of- a) contacting an alkanoic acid or ester of the formula R 2 -CH 2 -COOR 3, wherein R 1 is as defined for the ethylenically unsaturated acid or ester above and R 3 is independently, hydrogen or an alkyl group with 1 to 12 carbon atoms, with a methylene or ethylene source of formula I, optionally in the presence of a suitable catalyst, where R 5 and R 6 are independently selected from C 2 -C 12 hydrocarbons or H; X is either O or S; n is an integer from 1 to 100; and m is 1 or 2; and optionally in the presence of an alkanol to produce an impure ethylenically unsaturated acid or ester of the formula:- R 2 -C(=(CH 2) m)COOR 2; b) purifying the impure product of step a) by contact with an acidic ion exchange resin until the ion exchange resin is at least partially deactivated; and c) treating the at least partially deactivated ion exchange resin with an alkanol of formula R 4 OH, wherein R 4 is an alkyl group with 1 to 12 carbon atoms, and a carboxylic acid of formula:- R 1 -CH 2 -COOH to thereby reactivate the acidic ion exchange resin and produce as a by-product an ester of formula:- R 1 -CH 2 -COOR 4; d) optionally hydrolysing the ester of step c) to produce an acid of formula:- R 1 -CH 2 -COOH; e) recycling the said ester or acid of step c) or acid of step d) as a reactant in step a) with the proviso that when the ester of step c) is recycled, R 4 and R 3 are the same alkyl group. a) contacting an alkanoic acid or ester of the formula R 2 -CH 2 -COOR 3, wherein R 1 is as defined for the ethylenically unsaturated acid or ester above and R 3 is independently, hydrogen or an alkyl group with 1 to 12 carbon atoms, with a methylene or ethylene source of formula I, optionally in the presence of a suitable catalyst, where R 5 and R 6 are independently selected from C 2 -C 12 hydrocarbons or H; X is either O or S; n is an integer from 1 to 100; and m is 1 or 2; and optionally in the presence of an alkanol to produce an impure ethylenically unsaturated acid or ester of the formula:- R 2 -C(=(CH 2) m)COOR 2; where R 5 and R 6 are independently selected from C 2 -C 12 hydrocarbons or H; X is either O or S; n is an integer from 1 to 100; and m is 1 or 2; and optionally in the presence of an alkanol to produce an impure ethylenically unsaturated acid or ester of the formula:- R 2 -C(=(CH 2) m)COOR 2; b) purifying the impure product of step a) by contact with an acidic ion exchange resin until the ion exchange resin is at least partially deactivated; and c) treating the at least partially deactivated ion exchange resin with an alkanol of formula R 4 OH, wherein R 4 is an alkyl group with 1 to 12 carbon atoms, and a carboxylic acid of formula:- R 1 -CH 2 -COOH to thereby reactivate the acidic ion exchange resin and produce as a by-product an ester of formula:- R 1 -CH 2 -COOR 4; d) optionally hydrolysing the ester of step c) to produce an acid of formula:- R 1 -CH 2 -COOH; e) recycling the said ester or acid of step c) or acid of step d) as a reactant in step a) with the proviso that when the ester of step c) is recycled, R 4 and R 3 are the same alkyl group.