A new laboratory procedure for the evaluation of the mean molecular weight (mean relative molecular mass) of petroleum oils with high accuracy is described. The density and dynamic viscosity of three commercial petroleum oils are measured at different temperatures. These experimental data are used to calculate the kinematic viscosity as a function of the temperature, a typical task in an introductory-level laboratory course in physical chemistry. This new procedure offers an alternative to the classic use of a capillary viscometer. The kinematic viscosity data are used to evaluate the mean molecular weight of the petroleum oils. A set of equations is used that fits the data provided by the ASTM standard D 2502-04. (This standard is commonly used in the petroleum industry to calculate the mean molecular weight of petroleum oils.) The results confirm that viscosity measurements obtained with the new procedure can be used to evaluate the mean molecular weight of petroleum oils with high accuracy. (Contains 1 table.)