The nonlinear diffusion equation has been solved numerically to examine the effects of the temperature dependences of the heat capacity and the thermal conductivity on the variation of temperature with time in heat‐pulse experiments. A range of dependences, simulating amorphous and crystalline insulators at low temperatures, and a range of temperature throws up to 25% have been used in the calculations. It is found that the use of the ordinary diffusion equation may introduce errors of up to 10% for throws of 10%. The effects are particularly noticeable if the thermometer is relatively close to the heater, and resemble those observed in recent low‐temperature experiments.