The effect of complexation with Lewis acids on the tautomeric equilibrium of a derivative of pyridoxal has been explored using density functional theory. Three complexation sites (pyridine nitrogen, aromatic ring, and carbonyl group) have been considered. The tautomeric equilibrium can be modulated with the formation of complexes at the different sites. The changes observed in the geometric, energetic and electronic properties of the tautomers and transition states have been analyzed. Relationships have been found between those parameters, including a relationship between energetic and geometric parameters in agreement with the Hammond postulate. Copyright © 2010 John Wiley & Sons, Ltd.