The ionization constants of 5-substituted 4-hydroxy-2-methylpyrimidines (1-11) and some of their methyl derivatives, 4-methoxy-2-methylpyrimidines, 1, 2-dimethyl-4 (1H)-pyrimidones and 2, 3-dimethyl-4 (3H)-pyrimidones were determined. From the ionization constants, the degree of lactim-lactam tautomerism in the 4-hydroxypyrimidines was estimated. It was found that each of the pyrimidines exist mainly as a mixture of lactam forms (4 (1H)-pyrimidone and 4 (3H)-pyrimidone) in approximately equal amounts, in aqueous solution (ca. 1×10^-4M). The results were also supported by ultraviolet absorption spectroscopy. The substituent effect on the tautomerism was discussed in connection with the Hammett equation for the basicity of the pyrimidines.