The objective of this study was to limit oxidative changes in fish oil, rapeseed oil, and in their mixtures. Three types of samples were analysed: 100 % fish oil, 100 % rapeseed oil, and their 50:50 mixture. Three types of packaging were used: without gas protection, in the N₂ atmosphere, and in the CO₂ atmosphere. During the first phase of the investigation, fresh samples were put through accelerated oxidation in the Rancimat apparatus. The induction time of fish oil was 0.90 h, of 50:50 mixture – 1.28 h, and of the rapeseed oil – 5.03 h. Next, the samples were tested using a thermostat. The peroxide and anisidine values were determined, and the TOTOX index of fat was computed for fresh samples and for samples after the seven days of investigations. It was proved that inert gases had a protective impact on samples under analysis. The best results expressed as a low peroxide and anisidine values of the samples analysed were obtained using CO₂ (LOO of fish oil increased by about 0.93 meq O₂/kg, and AV increased by about 8.11).