A series of substituted N,N-dimethyl-N'-pyridylformamidines has been synthesised and the pK, values of the conjugate acids have been measured in water at 25°C. Consideration of the pK, values shows that initial protonation is predominantly on the imino nitrogen of the amidine system rather than on the pyridyl nitrogen. The effect of the protonation on the 'H and 13C NMR spectra of the amidines is discussed.In general, protonation of the amidine functional group N=C-N occurs on the imino nitrogen (la) rather than the amino l a l b .N-CHa I CH, X 2 5Me 3 4-Me