Continuously stirred tank reactors can make transitions between stationary states, from stationary to oscillatory stales, and vice versa. With the aim of characterizing these transitions thermodynamically, we calculate for some specific models the entropy increment due to mixing of the inflow into the reactor contents and the entropy production due to chemical reaction. These thermodynamic quantities are, however, found not always appropriate in describing the transitions, To explore another possibility, we also evaluate the thermodynamic force and flux of chemical reaction. Ii is shown as a result that spontaneous transitions are invariably accompanied by a decrease in the thermodynamic force or flux or both; i.e., the effects of irreversible processes relax at least in terms of the thermodynamic force or flux for nonequilibrium transitions. The same type of regularity is observed when the residence time is lengthened in stationary states.