In this work, a semitheoretical model based on the momentum transport theory is generalized through the development of a group contribution method. Viscosity data for temperatures between 13.95 and 950 K and pressures from 1.00 × 10–3 bar up to 1.00 × 104 bar were used to develop and to validate the model. To perform viscosity calculations, the model requires density and residual enthalpy that were obtained from a modified Peng–Robinson equation of state. Also, the model has one adjustable parameter that can be estimated form the group contribution method. Viscosity was calculated for 256 substances including 95 non-polar and 161 polar compounds. In total, 23 organic families were considered. Average deviations below 6.67% were calculated. The performance of the model was compared with those of other models reported in the literature. Calculations were carried out to include the saturation and single-phase regions. In general, it can be considered that the proposed model describes viscosity adequately using only one adjustable parameter.