The reaction of N2O5 with water vapor, a potentially important nighttime loss reaction for NOx and formation pathway for nitric acid, has been investigated at 298≠1 K in two environmental chambers of 3800 and 5800 L volume. From the N2O5 decay rates as a function of water vapor concentration, an upper limit to the N2O5+H2O rate constant of less than 2.4¿10-21 cm3 molecule-1 s-1 is derived. However, while gas phase nitric acid was observed to be a major product, its yield, especially at low water vapor concentrations, was significantly lower than the value of 2 expected, indicating that the overall reaction in these chambers involved both homogeneous and heterogeneous components. After allowing for the contribution of the heterogeneous reaction, an estimate of the gas phase homogeneous rate constant for N2O5+H2O of 1.3¿10-21 cm3 molecule-1 s-1 was obtained. While this value must still be regarded as an upper limit to the homogeneous rate constant, it is a factor of 10 lower than the only directly measured upper limit reported previously.