The structures of novolac monoalkyl(C8–C12)phenolformaldehyde oligomers modified with imidazolines based on fatty acids of sunflower or corn oils and polyamines – diethylenetriamine, triethylenetetraamine, polyethylenepolyamines were investigated by IR-spectroscopy. As a result of the analysis of the initial and final products, a reaction mechanism is proposed. Optical densities of the absorption bands of the main functional fragments were calculated, and the regularities of the changes depending on the qualitative and quantitative composition of the oligomers were studied.