So-called isothermal calorimeters are often built on the heat conduction principle. This may sound as a contradiction, as heat conduction is a result of non-isothermal conditions, but most measurements with microcalorimeters and other such instruments are essentially isothermal as the temperature differences within the instruments are in the order of 10 InK. This paper presents simple methods to estimate the temperature changes within a sample in a heat conduction calorimeter during a measurement. It also shows that the determination of activation energy is normally not disturbed by these small temperature changes. In addition to the heat production rate in the sample, the thermal properties of the sample and the calorimeter are needed as input to the calculations.