The application of the thermodynamical law of mass action to the calculation of equilibrium conduction electron and hole concentrations in semiconductors is explained. The method can be applied to arbitrarily complicated systems of donors, traps, acceptors, etc., in addition to the intrinsic electron-hole excitation. The discussion is limited to small defect concentrations and quasifree charge carriers, so that the ideal-gas free energy and the Sackur-Tetrode entropy constant can be used. Exact solutions are straightforward but often so complicated that simplifying assumptions are useful. As an example the effect of compensation of donors is calculated as a function of temperature. The Fermi energy is simply related to the electron concentration.