The possibility of group identification of organic compounds, specifically their assignment to the corresponding homologous series) on the basis of differential quantities, such as differences in their physicochemical characteristics or analytical parameters, is considered. The first example of using difference in the refractive indices at different wavelengths for the determination of the overall concentration of aromatic hydrocarbons in gasoline fractions is described. Various methods for the assignment of unknown compounds to homologous series are analyzed, in particular those based on the limiting values of refractive indices and relative densities, differences in gas chromatographic retention indices on stationary phases of different polarities, combinations of gas chromatographic retention indices with partition coefficients for heterogeneous organic solvent systems, and two versions of combinations of mass spectrometric and GC parameters, namely molecular retention indices and homologous increments of retention indices. The latter version (as the most important) is illustrated in detail by a particular example.