The behavior of the entropy of stationary states in a continuously stirred tank reactor (CSTR) operated by use of ideal gases is investigated in an attempt to gain insight into the thermodynamics of open nonlinear systems. The second law of thermodynamics predicts that for an adiabatic CSTR, the change of entropy of the reactor contents in a stationary state relative to the inflow is always nonnegative. This is an example of the law of entropy increase in an isolated system. This law as such does not in general apply to a nonadiabatic CSTR, i.e., when the reactor exchanges heat with its environment. The law, however, can still be made valid for such systems under suitable conditions provided the entropy of a stationary state is measured relative to the equilibrium state that would be attained by isolating the stationary state from its surroundings. This is illustrated on the basis of a number of specific models.