We report the results of a density functional theory (DFT) study of the structure and synthesis of a formazan molecule, 1,3,5-triphenylformazan. Three conformational minima of this formazan are identified, with the global minimum having an internal H-bond as part of a pseudo-six-membered ring. At the 6-31G(d) B3LYP level of theory the transition state for tautomerization and exchange of the hydrogen ion lies 4 kcal mol(-1) above the global minimum. This barrier height is unchanged by the incorporation of solvent models. An extensive investigation of the potential energy surface for the synthesis of 1,3,5-triphenylformazan from benzaldehyde N-phenylhydrazone and benzenediazonium ion is reported. The results support the mechanism whose first step is the addition of the diazonium onto the central carbon atom of the phenylhydrazone. However, the transition states for both this addition and addition to the amino nitrogen atom lie on intrinsic reaction paths which connect to a minimum of the phenylhydrazone which is not its global minimum. The substitution of a methyl group for the hydrogen on the amino nitrogen of the hydrazone raises the energy of the initial intermediate by 10 kcal mol(-1), preventing formazan formation. The increase in energy of the intermediate corresponds with an increase in energy (relative to that of the unmethylated case) necessary to place the benzaldehyde N-methyl-N-phenylhydrazone into the right conformation for reaction. The inclusion of a solvent model of ethanol made negligible difference to the computed geometric structures, but significant changes in energies relative to those of the reactants. However, adding self-consistent reaction field (SCRF) solvation energies to the gas-phase free energies gave relative values that were similar to those obtained by full inclusion of solvent effects (SCRF) in the optimization and vibrational frequency computations.