The study of oxidation kinetics of isooctyl-3-(3,5-di-tert-butyl-4- hydroxyphenyl)propionate (Irganox 1135) stabiliser was carried out using silicone oil as an inert substrate. The stabiliser was exposed at 80-120 °C under 0.02-3.0 MPa oxygen pressure. UV/VIS spectrophotometry, gel permeation chromatography and high pressure liquid chromatography were used to follow the stabiliser consumption, whose kinetic parameters were determined by applying a simple kinetic model.