The keto-enol tautomerism of 4-hydroxypyrimidine and of the related molecules S-methyl-2-thiouracil and 2-thiouracil has been investigated using synchrotron based techniques. The populations of the constituent tautomers and thermodynamic parameters have been obtained by analysis of core level photoemission spectra. The effect of substituents on the stability of tautomers has been revealed. Attaching additional OH (or SH) groups to the aromatic ring, stabilizes the dioxo (or oxo-thione) forms. However, substitution of hydrogen in position 2 by an S-CH3 group (that is, in going from 4-hydroxypyrimidine to S-methyl-2-thiouracil), does not significantly affect the tautomeric equilibrium.