AbstractCapillary viscometry has been employed to measure the viscosities of dilute polymer solutions over the temperature range ‐10 to 150 °C. A Group II base oil containing 95% saturates was used as solvent for an olefin copolymer (OCP), a hydrogenated diene copolymer (HDP), and a polymethacrylate (PMA). These three polymers represent the three major families of viscosity index (VI) improvers used nowadays in lubricant formulations. Intrinsic viscosities and Huggins' constants were also determined. The thickening effects of the olefin copolymer and the hydrogenated diene copolymer were found to be higher at low temperatures (e.g., 40 °C) than at higher ones (e.g., 100 °C), which phenomenon was attributed to stronger intermolecular hydrodynamic interactions at low temperatures, as indicated by the Huggins constants. For the hydrogenated diene copolymer and the polymethacrylate polymer, the viscosity increased abruptly when the temperature went below 10 °C. This unusual observation was attributed to the crystallisation of a small fraction of the base oil. Based on the intrinsic viscosity data, it was concluded that at temperatures between 10 and 150 °C, the polymer coil dimension remains a constant for the olefin copolymer and the hydrogenated diene copolymer VI improvers, but increases with increasing temperature for the polymethacrylate VI im