Abstract Mixed carbide fuel samples irradiated in various types of capsules were investigated with respect to fuel swelling and fission gas behaviour. The irradiations were carried out in the FR 2 reactor in Karlsruhe at temperatures between 300 and 1750°C up to 5.5% burnup. The swelling was evaluated by immersion density measurements in carbon tetrachloride. The fission gas determinations were carried out by measuring the released gas and by measuring the retained fission gas. The swelling rate of mixed carbide is a strong function of temperature. At temperatures below 1000°C it is between 1 and 1.5% per % burnup. At temperatures above 1000°C the swelling rate increases with temperature. It is about 3% per % burnup at 1300°C and about 12% per % burnup at 1750°C. The swelling rate at high temperatures decreases with increasing burnup due to a saturation of the fission gas bubble porosity.