In this work, we have investigated the monovariate relationship between reduced viscosity and residual entropy in pure fluids and in binary mixtures of hydrocarbons and hydrocarbons with dissolved carbon dioxide. The mixtures considered were octane + dodecane, decane + carbon dioxide and 1,3-dimethylbenzene (m-xylene) + carbon dioxide. The reduced viscosity was calculated according to the definition of Bell, while the residual entropy was calculated from accurate multi-parameter Helmholtz-energy equations of state and, for mixtures, the multi-fluid Helmholtz energy approximation. Mono-variant dependence of reduced viscosity upon residual molar entropy was observed for the pure fluids investigated and, by incorporating two scaling factors (one for reduced viscosity and the other for residual molar entropy), the data were represented by a single universal curve. To apply the method to mixtures, the scaling factors were determined from a mole-fraction weighted sum of the purecomponent values. This simple model was found to work well for the systems investigated. The average absolute relative deviation (AARD) was observed to be between 1 % and 2 % for pure components and a mixture of similar hydrocarbons. Larger deviations, with AARDs of up to 15 %, were observed for the asymmetric mixtures but this compares favourably with other methods for predicting the viscosity of such systems. We conclude that the residualentropy concept can be used to estimate the viscosity of mixtures of similar molecules with high reliability and that it offers a useful engineering approximation even for asymmetric mixtures.