The thermal history and the chemical differentiation of a porous H2O-CO2-ice sample is studied for four different examples. In the first two cases, the possible effect of a nonvolatile mantle covering the ice is investigated. In examples 3 and 4 the influence of the average grain size and the Hertz factor, which determine the effective thermal conductivity of the ice, on the sample evolution are investigated. Our computations show that both the amount of heating and the compositional evolution critically depend on the assumed ice material parameters and the presence of a mantle. The results are compared with other theoretical work and relevant experiments, and we found good agreement between experimental results and our theoretical predictions. Finally, some implications for comet nuclei and future spacecraft missions to comets are discussed. ¿ American Geophysical Union 1992