This invention relates to a process for the production of lower aliphatic esters by reacting a lower olefin with a saturated lower aliphatic mono-carboxylic acid in the vapor phase in the presence of a heteropolyacid catalyst characterized in that an amount of water in the range from 1-10 mole % based on the total of the olefin, aliphatic monocarboxylic acid and water is added to the reaction mixture during the reaction. The presence of water enhances the yield of the desired ester. The reaction mixture may optionally contain a di-ether to minimize the by-products.The process is particularly suitable for producing ethyl acetate from ethylene and acetic acid in the presence of water.