The question of the quantitative energetics of organic reactions in solids is addressed by studies of the solid state reaction of 2,5-distyrylpyrazine (DSP). Crystal electronic spectra permit the role of excitons to be analyzed. Experiments are described which elucidate the evolution of lattice vibrational modes as a function of extent of the reaction and the relative motions of the molecules associated with those modes. A general theory of solid state reactivity is employed to interpret the structural and spectroscopic results to produce a coherent, comprehensive picture appropriate for solid state reactions