The thermal influence of lithic clasts in mixtures of hot gases and ejecta produced during explosive eruptions is an important aspect of the dynamics of eruption columns and pyroclastic flows. In order to study this aspect we present a heat conduction model, which considers both the temperature distribution of the lithic clasts and the exchange of thermal energy between the lithic clasts and other consitituents (gases and juvenile pyroclasts). The model quantifies the thermal energy of the system by means of a characteristic energy function that describes the energy absorbed by each lithic clast. Using this energy function and considering some hypothetical arrangements of lithic clasts it is possible to calculate, by means of heat balance, the temperature variations in mixtures of hot gases and ejecta as a function of time, volume fraction of particles to gas, and thermal diffusivity, initial temperature, diameter, and volume fraction of lithic clasts. Theoretical results show that in an eruption a volume fraciton of 12% of cool lithic clasts (20--100¿C) whose size is less than 5 cm will cause the mixture of hot gases and ejecta to cool by 300¿C. We suggest therefore that the thermal effects of lithic clasts should be considered in numerical models of eruption columns and pyroclastic flows. ¿American Geophysical Union 1991