The absorption of light by some but not all species of a chemical reaction, followed by a radiationless transition and ultimate conversion of light into heat on a time scale short compared to the chemical reaction time scale, is shown to give rise to the possibilities of multiple steady states, damped oscillations in state variables, hysteresis, and instabilities. All these phenomena are predicted to occur even for the simplest reaction A⇄B, where only A absorbs light, and where the rate equation, with temperature dependent rate coefficients, is coupled nonlinearly to the equation for the rate of change of temperature. The theory is developed for both stationary and transient experiments. For the cyclic reaction mechanism A⇄B⇄ C⇄A, where again only A absorbs light, damped oscillations occur under isothermal conditions; the illumination, as described, effectively breaks microscopic reversibility. Both the kinetic and the thermodynamic analysis show the essential role of light in effectively breaking microscopic reversibility analogous to the net flux of reactants and of products across the boundary of an open system. In nonequilibrium relaxation experiments performed on illuminated systems with damped oscillations, both a frequency and a decay rate may be measured. The application of periodic perturbations leads to resonance effects.