By ¹H, ¹³C and ¹⁵N n.m.r. studies we have established that tautomeric equilibria of 9-formyl-tetrahydro-4H-pyrido[1,2-a]pyrimidines and their homologues are controlled mainly by the size of the ring containing a single nitrogen atom. Thus in the pyrrolo analogues the enol imine form is predominant, with the azepino homologues the enamine, and with the azocino analogues the imine. The enol imineâ enamine interconversion is relatively fast, whereas the imineâ enamine interconversion is slow on the n.m.r. time-scale. The relative stabilities of the individual tautomers are explained by stereo-chemical factors and hydrogen bonding.