Abstract: The present invention relates to the carbonylation of an ester, specifically vinyl acetate. The process comprises the carbonylation of vinyl acetate comprising reacting vinyl acetate with carbon monoxide in the presence of a source of hydroxyl groups and of a catalyst system, the catalyst system obtainable by combining: (c) a metal of Group VIII B or a compound thereof: and (d) a bidentate phosphine of general formula (I). A process for the production of a lactate ester or acid of formula (II), comprising the steps of carbonylating vinyl acetate with carbon monoxide in the presence of a source of hydroxyl groups and of a catalyst system is also described. A process for the production of 3-hydroxy propanoate ester or acid of formula (III) comprising the steps of: carbonylating vinyl acetate with carbon monoxide in the presence of a source of hydroxyl groups and of a catalyst system also forms an aspect of the invention.