Several mechanisms for trans to cis isomerization for diazene (N2H2) were considered at reasonably high levels of ab initio theory. Inversion and N-H bond scission have activation barriers much too high to explain the rapid trans --> cis isomerization which must precede the facile dihydrogen transfer from the higher-energy cis isomer in solution. While one catalytic water has little effect on the activation barrier, two catalytic waters reduce the activation barrier to 26.9 kcal/mol (in the gas phase). When solvation effects are considered by combining AM1-SM2 solvation energies with ab initio free energies, one mechanism appears to be consistent with experimental observations. The solution phase proton affinity of trans-N2H2 is 3.2 kcal/mol smaller than that for H2O. This result indicates that trace amounts of acid can rapidly protonate trans-N2H2 to form N2H3+. In turn, N2H3+ can lose a proton to form either the cis or the trans isomer in an equilibrium ratio determined by the free energy difference, which is calculated to be 4.9 +/- 2.1 kcal/mol at 298 K.