The viscosity model is an important component in enhanced oil recovery packages and, for pure bitumen, several accurate models are available. In this study, a simple correlation presented in an earlier publication is extended to predict the viscosity of bitumen-diluent mixtures, as well as the mass fraction required to reduce bitumen viscosity to pumping viscosity. In developing the viscosity model, viscosities of pure bitumen and diluent were used as the endpoints. and the diluent mass fraction was raised to a power of "n" (a viscosity reduction parameter) to account for the sharp drop in bitumen viscosity with increase in diluent mass fraction, The model was developed with 99 data points from three different bitumens and five diluents; spanning a viscosity range of 10(-1) to 10(6) mm(2)/s. The model was used to recalculate the viscosity and mass fraction values, and results compared with similar correlations by Cragoe and Chirinos. The best match was obtained with our correlation, with overall average absolute deviations of 12% and 5% for viscosity and mass fraction predictions, respectively. Predictions on data not used in developing the model showed an excellent match between experimental and predicted values, with an overall average absolute deviation of below 10% for viscosities of mixtures at 15 degrees C. 60.3 degrees C. and 83.6 degrees C.