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Analysis of the optical response of a molecular crystal undergoing an external, periodic modulation is presented in terms of classical dipole theory. The case of modulated strain is treated in particular and identification of such strain with acoustic phonons with q=0 permits use of lattice dynamical theory to elucidate the acoustic phonon–exciton interaction. It is shown that extrema in the piezomodulated reflectivity (PMR) locate transverse and longitudinal exciton–polariton frequencies under specific experimental conditions. Model calculations demonstrate the great difference in the PMR for strong and weakly coupled systems. Relation of the PMR intensity to the elasto-optic coefficients and the storage of mechanical energy by the crystal permits extension of the technique to the study of such phenomena as internal strains and phase transitions where the partition of the mechanical energy among the various crystal excitations is of interest.