Thermal variability of the coefficient of the critical state, or the angle of internal friction is examined in view of experimental results obtained for clays and hard rocks. This sensitivity has several implications for the evolution of what is considered as apparent preconsolidation pressure in constant stress heating tests. Failure conditions in soils at elevated temperatures appear to be strongly dependent on the history of application of stress and temperature, and hence are affected by the thermal sensitivity of the internal friction angle. Consequences to ranges of admissible processes are discussed.