Abstract: An improved process for producing ethylenically unsaturated phosphate ester compounds, especially 2-(methacryloyloxyethyl)-2' (trimethylammoniomethyl) phosphate, (HEMA-PC) comprises a two step reaction in the first of which a phospholane reagent is reacted with a hydroxyl containing ethylenically unsaturated starting material, and in the second of which the intermediate formed in the first step is ring opened by reaction with trimethylamine to produce a zwitterionic product. In the process the two steps of the reaction are carried out in the same solvent with substantially no removal or addition of solvent after the first step. The solvent is preferably acetonitrile. The use of pure reagents avoids the production of undesirable by-products. The product is suitable for polymerising, for instance to provide cross-linked hydrogel copolymers used in ophthalmic applications.