AbstractViscosity modifiers are one of the most important components of multi‐viscosity grade lubricants. In use, the lubricant heats up and the ingredients degrade and deplete. In an attempt to understand the thermal and thermal‐oxidative behaviour of viscosity modifiers, we have studied the effects of heat in both inert and oxidative atmospheres. The paper describes the degradation of a styrene‐isoprene viscosity modifier studied with differential scanning calorimetry (DSC). Most of the kinetic parameters can be obtained by the application of Kissinger, Ozawa, and Augis and Bennett's simple expressions, which are special cases of Elder's formulations. The values of order, heat of degradation, and activation energy were +1.25, +1149 J·m, and 175 kJ·mole for thermal degradation in a nitrogen atmosphere, and +1.50, −6321 J·m, and 57 kJ·m for thermal‐oxidative degradation in air. The information concerning the degradation of the viscosity modifying polymers can be useful in selecting the correct polymers for envisaged application.