The method of Lohrenz-Bray-Clark, which relates the residual viscosity to the reduced density, is the most widely used engineering tool to predict the viscosity of reservoir fluids. Systematic investigation of the viscosity of pure compounds at various pressure and temperature conditions indicated the requirement of including the structural and thermal effects for accurate viscosity prediction of dense fluids. The method has been modified to include the above-mentioned effects. Reliability of the modified method for calculating viscosity of mixtures and in particular of high-pressure high-temperature fluids has been demonstrated. © 2001 Elsevier Science Ltd. All rights reserved.