Values of both self-association and protomeric equilibrium constants are reported for 2-hydroxypyridine-2pyridone, 4-hydroxypyridine-4-pyridone, and 2-mercaptopyridine2-thiopyridone isomer pairs in different solvents. These results provide quantitative evidence for significant differences in the positions of protomeric equilibria for self-associated and monomeric species. The 2-substituted isomers are associated as well-known dimers while the 4-substituted systems form oligomers. In polar and hydrogen-bonding solvents self-association is substantially reduced. Sterically hindered 2-and 4-pyridones are less associated than unhindered systems. The implication of these results, that determinations and interpretations of protomeric equilibrium should take into account the possible dominance of self-association, is discussed. H 1 , X = Y = Z = H 2, X = Y = Z = H 3, X = C1; Y = Z = H 4, X = C1;Y = Z = H 5, X = CH,;Y = OCH,; Z = H 6, X = CH,; Y = OCH,; Z = H 7 , X = CH,; Y = H; 8, X = CH,; Y = H; 9, X = CH,; Y = H; 1 0 , X = CH,; Y = H; Z = CH=CH(CH,),CH, Z = CH=CH(CH,),CH, CH(CHz) 3CH3

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