Treatment of PtCl4 with CO at 40-110 degrees C forms a powerful alkyne hydration catalyst that operates both under homogeneous conditions in wet THF and under phase-transfer conditions in (CHCl2)(2)/H2O in the presence of tricaprylmethylammonium chloride (Aliquat 336). Complex HPtCl(CO)(2) is regarded as the active hydration catalyst. It is assumed to be formed by initial transformation of PtCl4 to H-2[Pt-3(CO)(6)](n), (n = 5, 6) followed by reaction with HCl (generated by decomposition of the starting platinum salt).