Reactions of the acetylene radical cation (C2H2+center dot) with H2O were investigated using ion mobility mass spectrometry. The primary products are the C2H3O+ and C2H4O+center dot ions, produced with an overall rate coefficient k(300 K) = 2(+/- 0.6) > 10(-11) cm(3) s(-1) that increases with decreasing temperature. The C2H4O+center dot (adduct) vs C2H3O+ (H loss) ratio also increases with decreasing temperature, and with increasing third-body pressure. Ab initio calculations on the products showed seven stable C2H3O+ isomers and eleven stable C2H4O+center dot isomers. In the C2H4O+center dot adduct channel, the reactivity and energetics suggest that the adduct is the H2C=CHOH+center dot (vinyl alcohol) ion. In the C2H3O+ channel, the H loss occurs exclusively from water. The C2H3O+ product ion undergoes slow deprotonation by water to form H+(H2O)(n), clusters. The reactivity, combined with energetics, suggests that the protonated ketene CH2COH+ is the most likely observed C2H3O+ ion probably with some contribution from the cyclic c-CH2CHO+ ion.