A formula is derived which expresses the rate of a chemical reaction (or relaxation process) as the convolution of a correlation integral and the isothermal fluctuation from equilibrium of the affinity of the reaction. Two examples are provided of the dynamic variables (reaction densities) that appear in this correlation integral. The first of these is the model of simple reactive spheres. The second incorporates mechanisms by which two colliding atoms can be transformed into a bound dimer and, conversely, by which dimer dissociation can occur. (C) 2003 American Institute of Physics.