Description of phase behaviour of a highly nonideal system mineral oil + DMF is a rather complex task, because hydrocarbon feed, which has to be separated into aromatic and nonaromatic fractions, consists of large number of compounds that differ significantly in molecular weight and chemical structure. Such a complex mixture was substituted by two pseudo-components, aromatic and nonaromatic in order to simplify the procedure. They were represented by adequate model compounds, whose thermodynamic behaviour was similar to the original one. The chosen model system simulates properly the real one in the sense of the mutual solubility of aromatic and nonaromatic components. However, this simplification of description of strongly nonideal mixtures should be considered as the first approximation.