UV-visible spectra were used to determine the acidity constants and the tautomeric properties of 1-substituted phenyl-3,5-diphenylformazans. The protonation and deprotonation behaviors of the compounds were investigated at different pHs. Protonation was observed for all compounds as two protons were acquired and deprotonation as one proton was lost. The effects of solvents on the tautomerism of 1-substituted phenyl-3,5diphenylformazans were also investigated in neutral, acidic, and basic solutions at different pH and temperatures. The tautomerism interconversion percentages for the studied formazans were calculated from the absorption spectra obtained for different solvents at various temperatures. It was observed that the chelate form of the studied formazans was the dominant structure. It was concluded that the type of the substituents and hydrogen bonding acceptor ability of the solvents were the significant factors for the interaction in tautomerism.