A study of the hydrolysis of (CO)(5)Cr=C(OMe)Ph (1a), (CO)(5)Cr=C(OEt)Ph (1b), (CO)(5)Cr=C(OMe)CH=CHPh (1c), (CO)(5)W=C(OMe)Ph (1d), and (CO)(5)W=C(OEt)Ph (1e) in 50% MeCN-50% water (v/v) at 25 degrees C is reported. The reaction occurs in two stages; the first is the formation of (CO)(5)M=C(O-)R or (CO)(5)M=C(OH)R (M = Cr or W, R = Ph or CH=CHPh) while the second, much slower stage is the formation of RCH=O and (CO)(5)MOH(-). This paper reports a kinetic investigation of the first stage. It is shown that nucleophilic attack by OH- at high pH and by water at low pH, presumably to form a tetrahedral intermediate, is rate limiting; in the presence of buffers general base catalysis of water addition is observed. Kinetic solvent isotope effects of ca. 1 for the OH- and of 3 to 4.7 for the water pathways are also reported. The results lead to insights regarding the following points : (1) effect of changing the metal (Cr vs W), the alkoxy group (MeO vs EtO), and the R group (Ph vs CH=CHPh) on reactivity; (2) relative reactivity of Fischer carbene complexes vs carboxylic esters; (3) reasons why a tetrahedral intermediate is not detectable even though the equilibrium for its formation is probably favorable at high pH; and (4) reasons why the hydrolysis of Fischer carbene complexes containing an acidic proton such as (CO)(5)Cr=C(OMe)CH3 follow an entirely different mechanism.