The ultraviolet (UV) spectra and the pK_a and pK_sp of 14 different variations of 4-hydroxy-1,3,3a,7-tetraazaindene (TAI) measured in water at 25°C are discussed in this report. The acidity of the tautomeric proton correlated with the inductive electronic effect of the substituent as measured by Hammett' σ_ρ constants; pK_a values obtained potentiometrically and spectrophotometrically ranged from 3.4 to 6.5. The solubility products of the 1:1 TAI:Ag salts were measured at equilibrium using a solid-state silver sulfide electrode. The pK_sp values ranged from 9.6 to 12.9 and increased with substituent bulk at the 2 position and with electron-withdrawing substituents at the 5 position. Oxidation of an alkylthio substituent at either the 2 or 5 position decreased the pK_sp value. A carboxylic acid group at the 2 position yielded a soluble silver complex with log β₁ of 7.9; however, carboxylic acid substitution at the 5 position yielded a sparingly soluble salt with pK_sp of 10.4. Values of pK_a and pK_sp were observed to be correlated only for substituents in definite homologous series; however, there was no general correlation within the 14 examples.