Substitution of protons in 12-tungstophosphoric, 12-tungstosilicic, and 12-molybdophosphoric acids with monovalent thallium cations, representative of group 3B in the periodic table, form high surface area solids which possess a micropore structure. Preparation of the salts with stoichiometric and nonstoichiometric quantities of the cation decreases the concentration of protons remaining in the solid and influences the interactions experienced by these residual protons. Solid state H-1 NMR and NH3 TPD measurements, correlated with catalytic activities observed in the isomerization of l-butene, have been employed to assess the number of acidic sites, the distribution of acidic strengths, and the effect of changes in the preparative stoichiometry in the three thallium 12-heteropoly oxometalate salts,