A new thermal analysis system has been designed to follow the influence of light on solid reactants. This method is based upon the association of a thermogravimetric system for the kinetic measuring method, and a U. V. irradiation system for the activation method. This instrument allows comparative studies of a thermal reaction and the corresponding photochemical one under exactly the same experimental conditions. The experiments, carried out in silver carbonate, have provided new information on photochemical kinetics, but also on solid-state reactivity and even on thermal processes. More generally, this device, designed for studies on reactivity, can be applied to any other system needing the action of light. With the device, the influence of the light intensity, the wavelength and the lifetime of light effects on many reactions can be studied.