N2H2 is an intermediate in the acid-assisted hydrolysis of azodiformate, (NCO2)22-. When the hydrolysis is conducted at pH almost-equal-to 4, the second-order dismutation of diazene occurs on a time scale longer than that of its generation. Due to this property, the intermediate (N2H2) and its decomposition can be observed directly in a stopped-flow instrument. The absorption maximum occurs at 355 nm with epsilon = 24 +/- 3 M-1 cm-1. The region of absorption (300-430 nm) is in fair agreement with that of the gas-phase spectrum reported by other workers. The rate constant for the second-order dismutation of N2H2 is (2.2 +/- 0.3) X 10(4) M-1 s-1 at 25-degrees-C and 0.11 M ionic strength, which agrees with the previously reported value (2.0 X 10(4) M-1 s-1) obtained by indirect methods. The temperature dependence of this process, over the temperature interval from 283 to 313 K, leads to the activation parameters DELTAH(double dagger) = 13.8 +/- 0.6 kJ mol-1 and DELTAS(double dagger) = -116 +/- 2 J K-1 moL-1. The dismutation displays an overall deuterium kinetic isotope effect of 3.3, as determined by conducting the reaction in a fully deuterated medium.