The processes which tend to dissipate the kinetic energy of interstellar particles are discussed, and the rate of energy loss for each process is derived in a form suitable for numerical computation of interstellar temperatures. The phenomena considered are: free-free emission by electrons in encounters with protons and H atoms; inelastic encounters of electronswjth atoms and with H2 molecules; excitation of rotational states of H2 molecules by encounters with H atoms and with H2 molecules; and inelastic coffisions of H atoms and H2 molecules with solid grains. Application of these and previous results to the analysis of interstellar temperatures will be made in a subsequent paper