Both theoretical and experimental investigations are reported for the gas-phase hydrolysis of the radical cation of ketene, H2CCO.+. Density functional theory (DFT) with the B3LYP/6-311++ G(d,p) method indicates that a second water molecule is required as a catalyst for the addition of water across the C=O bond in H2CCO.+ by eliminating the activation barrier for the conversion of [(H2CCOH2O)-H-.](.+) to [H2CC(OH)(2)](.+). Theory further indicates that [H2CC(OH)(2)(H2O)-H-.](.+) may recombine with electrons to produce neutral acetic acid. Experimental results of flow-reactor tandem mass spectrometer experiments in which CH2CO.+ ions were produced either directly from ketene by electron transfer or by the chemical reaction of CH2.+ with CO are consistent with formation of an (C-2, H-4, O-2)(.+) ion in a reaction second-order in H2O. Furthermore, comparative multi-CID experiments indicate that this ion is likely to be the enolic CH2C(OH)(2)(.+) cation. The results suggest a possible mechanism for the formation of acetic acid from ketene and water on icy surfaces in hot cores and interstellar clouds.