Determination of malondialdehyde is a widely used procedure for measurement of lipid peroxidation. In this paper we report an unusual temperature dependence of malondialdehyde formation in egg yolk phosphatidylcholine liposomes oxidized by the Fenton system (0.1 mmol/1 FeSC>4 and 0.05 mmol/1 H2O2). The amount of malondialdehyde formed was 37 % higher in samples kept at 22 °C than at 50 °C. An alternative method for determination of lipid peroxidation, measurement of oxygen uptake, revealed complete consumption of dissolved oxygen to peroxidized lipids at 22 °C as well as 50 °C. Since oxygen is essential for the formation of cyclic peroxides - precursors of malondialdehyde - we conclude that the nature of the observed effect consists in limitation of oxygen availability at elevated temperatures.