Ionization of N2O4 in and on thin water films on surfaces is believed to be a key step in the hydrolysis of NO2 which generates HONO, a significant precursor to the OH free radical in the lower atmosphere. Molecular dynamics simulations using "on the fly" high-level MP2 potentials are carried out for ONONO2 center dot(H2O)(n) clusters, n <= 8, used to mimic the surface reaction, in order to investigate the ionization process and determine its time-scale and mechanism around room temperature. The results are (i) the isolated molecule does not convert to the NO(+)NO3(-) ion pair, even for long times; (ii) for ONONO2 center dot(H2O)(n) with n = 1 and 2, ionization takes place in several picoseconds; (iii) for n >= 3, ionization is essentially immediate, implying that the neutral species does not have sufficient lifetime to be considered a significant intermediate in the reaction; and (iv) even at ice temperatures, T <= 250 K, ionization for n >= 3 is immediate. The implications for hydrolysis of oxides of nitrogen on surfaces in the atmosphere are discussed.