Infrared investigations on the v(NH) and v(CN) vibrations of N,N′-disubstituted formamidines, acetamidines and benzamidines, containing the same or different groups at the amino and imino nitrogen atoms, and of some model compounds (N-monosubstituted amides, imidazoles and 2-aminopyridines), carried out in CCl₄ at various temperatures and concentrations, and in other solvents, show that all the amidines studied have the E configuration on the CN double bond and display conformational isomerism on the C—N single bond. Formamidines exist predominantly in the form of a conformer with a hydrogen atom synperiplanar to the imino nitrogen atom, and can form a cyclic dimer. In acetamidines and benzamidines the other conformer, with a hydrogen antiperiplanar to the imino nitrogen, prevails, and hence they can form mainly linear dimers. Different positions of the v(NH) and v(CN) bands observed for N,N′-diaryl and N,N′-dialkyl derivatives permit determination of the composition of tautomeric mixtures in N-alkyl-N′-arylamidines.