Abstract: A thermodynamic model is formed which allows determination of the temperature at which waxes or paraffins appear in petroleum fluids, and notably in crudes, as well as the solid fraction that precipitates when the temperature of the petroleum fluid falls below this critical value. A differentiation is established between n-paraffins, iso-paraffins, naphthenes and aromatics. The model utilizes an analytical representation of the fluids by pseudo-components, the physico-chemical parameters of most of them being determined by combination of the corresponding parameters of a certain number of pure hydrocarbons gathered in a database. The model takes account of the non-ideality of the solid, liquid, and gas phases. Two of the pseudo-constituents including the heaviest fractions are represented by two ficticious molecules, each being defined by a molar distribution among various groups which constitute them, and a group contribution method is used to calculate the thermodynamic properties thereof.