The mechanism and kinetics for the gas-phase hydrolysis of N2O4 isomers have been investigated at the CCSD (T)/6-311++G(3df,2p)//B3LYP/6-311++G(3df,2p) level of theory in conjunction with statistical rate constant calculations. Calculated results show that the contribution from the commonly assumed redox reaction of sym-N2O4 to the homogeneous gas-phase hydrolysis of NO2 can be unequivocally ruled out due to the high barrier (37.6 kcal/mol) involved; instead, t-ONONO2 directly formed by the association of 2NO(2), was found to play the key role in the hydrolysis process. The kinetics for the hydrolysis reaction, 2NO(2) + H2O <-> HONO + HNO3 (A) can be quatitatively interpreted by the two step mechanism: 2NO(2) -> t-ONONO2, t-ONONO2 + H2O -> HONO + HNO3. The predicted total forward and reverse rate constants for reaction (A), k(tf) = 5.36 x 10(-50)T(3.95) exp(1825/T) cm(6) molecule(-2) s(-1) and k(tr) = 3.31 X 10(-19)T(2.478) exp(-3199/T) cm(3) molecule(-1) s(-1), respectively, in the temperature range 200-2500 K, are in good agreement with the available experimental data.