[en] A detailed treatment of the kinematics of relativistic systems of particles and photons is presented. In the case of a relativistic Maxwell-Boltzmann distribution of particles, the reaction rate and luminosity are written as single integrals over the invariant cross section, and the production spectrum is written as a double integral over the cross section differential in the energy of the produced particles (or photons) in the center-of-momentum system of two colliding particles. The result is valid for all temperatures and for the general case when the plasma contains different mass particles. The results are applied to the calculation of the annihilation spectrum of a thermal electron-positron plasma, confirming previous numerical and analytic results. Relativistic thermal electron-ion and electron-electron bremsstrahlung are calculated exactly to lowest order, and relativistic thermal electron-positron bremsstrahlung is calculated in an approximate way, by interpolating between the modified Bethe-Heitler formula and the exact lowest order electron-electron bremsstrahlung spectrum. An approximate treatment of relativistic Comptonization is developed. The question of thermalization of a relativistic plasma is also considered