Edible oils are recognised as essential nutrients in the human diet as they are one of the concentrated sources of energy that provide essential fatty acids, the building blocks for hormones needed to regulate body systems. The intense frying of oils causes thermal reactions, including hydrolysis, oxidation, polymerisation and isomerisation, resulting in thermal degradation of the oils. Heating stimulates the formation of peroxides, triacylglycerol and carbonyl compounds, which decrease the nutritional value of the oils. This leads to destructive diseases such as cardiovascular diseases, colon cancer, atherosclerosis and others. In the present study, palm and groundnut oils are subjected to four cycles of heating to frying temperature, and changes in their composition are studied using gas chromatography-mass spectrometry (GC-MS). The variation of the oil viscosity with heating time is also observed and is found to correlate with the composition of fatty acids in the oils.