The effect of dissolved gases on the viscosity of heavy oils has been modelled using a generalized correlation. The viscosity data for three heavy crude oils and crude oil-gas mixtures were obtained using a PVT-cell equipped with an on-line capillary tube viscometer. The measurements were conducted at various temperatures and pressures; and the viscosities calculated from the data using the Hagen-Poisseuille pipe flow equation. In this paper it is shown that the complex effect of three parameters, (temperature, pressure and dissolved gas concentration) on heavy oil viscosities can be accurately predicted by the correlation which requires only one viscosity measurement at a reference condition of 30 degrees C and one atmosphere. The correlation has been found to be successful for the following range of conditions : temperature, 28 degrees C to 110 degrees C; pressure, 0.8 MPa g to 20.7 MPa g; and viscosity, 0.01 to 1.7 x 10(6) Pa s. The average absolute deviations obtained with measured and curve-fit values at 30 degrees C are 10.4 per cent and 7.9 per cent, respectively.