Measurements of the heat of formation of N2H2from the appearance potential form/e30 ions from N2H4give irreproducible results. Using N2H2, reproducible values for the appearance potentials ofm/e30, 29, and 28 ions were obtained. These values yield an ionization potential of N2H2, IP(N2H2) = 9.7 ± 0.1 V, a heat of formation of N2H2, ΔHf(N2H2) = 36 ± 2 kcal/mol and a proton affinity of N2, PA(N2) = 5.6 eV. An estimate of the ionization potential of N2H radical, IP(N2H) = 7.6 V, was also obtained.