Chemical quasi-equilibria represent the product composition of stable components at the outlet of a nonisothermal plasma chemical reactor under operating conditions of very high or vanishing electric power input. The properties of these quasi-equilibria in relation to the thermodynamic equilibrium are discussed first of all using an instructive kinetic model. The investigations show that for the formation of the quasi-equilibria the existence of two time scales for the chemical conversions in the system is necessary. The quasi-equilibrium states are formed on a short time scale by fast relaxation and deactivation processes of unstable components. After that the system passes into the thermodynamic equilibrium for very long times. The transition from a chemical quasi-equilibrium state into the thermodynamic equilibrium was detected in the H-2-I-2-HI system experimentally, too. The analysis of a detailed microphysical model for this system shows that the transition into the thermodynamic equilibrium is caused by the increasing thermal production of unstable components (e.g. iodine atoms) at higher gas temperatures. Under these conditions the electronic production of atoms is only a small correction of the thermal degree of dissociation, and the thermodynamic equilibrium is the only stationary state for the system.