A new theory of the time-dependence of diffusion-controlled reactions in fluids and solids near thermal equilibrium is presented. Quite general reaction schemes can be handled, but the treatment is limited to the case of particle-conserving unimolecular and bimolecular reactions. Mori’s projection operator formalism, as applied to a general master equation with detailed balance, and Ursell’s cluster expansion are used as tools. The theory is applied to the time-dependence of fluorescence in a simple model without hydrodynamic or potential interactions.