The factors that influence the C=N stretching frequency and the C=N pi-bond strength of unprotonated and protonated imines have been studied using generalized valence bond (GVB) calculations. For simple imines, such as, CH2=NH and trans-CH3CH=NCH3, the C=N stretching force constant increases upon protonation, whereas for polyimines (CH2=(CHCH=)(n)NH), the C=N stretching force constant decreases by 0.47 mdyn/Angstrom upon protonation. The calculations clearly show that the C=N(H) stretching frequency and its deuterium isotope shift in protonated imines are significantly influenced by the C=NH bending and the C=N/C=NH coupling force constants. The increase in the C=N(H) stretching frequency upon protonation is shown to be neither due to a negative C=N/C=NH coupling force constant nor due to the C=N stretching but to be mainly due to the strong contribution from the large C=NH bending force constant (1.08 mdyn/rad(2)) and a positive coupling constant (0.30 mdyn/rad). This C=NH bending contribution also affects the C=N(H) stretching frequency deuterium isotope shift. The GVB calculations predict a deuterium isotope shift of 26 cm(-1) in the C=N(H) stretching frequency and a N-15 isotope shift of 18 cm(-1), which are in excellent agreement with the corresponding experimental values of 25 and 15 cm(-1), respectively.