The hydration of ethylene was carried out over different metal phosphates and phosphoric acid impregnated metal phosphates at high pressure. The catalysts were compared to phosphoric acid on silica, which is used in the industrial application of the reaction. The impregnated phosphates showed slightly higher conversions than phosphoric acid on silica. It was shown by X-ray diffraction that aluminum and zirconium phosphate are, like silica gel, not inert against phosphoric acid. The activity of the catalysts, the amounts of leached phosphoric acid, the determination of the surface areas, of the free phosphoric acid, and of the acidity indicated that the catalytically active compound is liquid phosphoric acid.