Abstract The Investigation of tautomeric equilibria among pyrimidyl-2-methanes has been undertaken. Pyrimidyl-pyrimidyledene equilibrium has been found in the case of 5-substituted 2-pyrimidyl-cyanoacetic esters. Unsymmetrically substituted pyrimidyl-2-methanes form two types (1-NH and 3-NH) of ylidene tautomers, which differ in ring proton spin coupling constants. The effect of substituents in the pyrimidine ring and of the solvent on tautomeric equilibrium are discussed. A drastic effect is produced by acids (CF 3 COOH) which shift the equilibrium toward the ylidene tautomer of higher basicity. The thermodynamic and kinetic characteristics and the mechanism of the tautomeric equilibrium were examined.