Abstract A theory is presented of suppression and enhancement of the exciton–phonon interaction, which strongly affects optical absorption spectra in microcrystallites. The motivation of the study comes from an experimental observation that the absorption bandwidth of aromatic microcrystallites increases with the crystallite size, but it shows a sudden fall to zero at a certain size. The present theory shows that the fall comes from the broadening of the exciton states due to phonon scattering, which is efficient only in crystallites larger than a critical size. The broadening results in a remarkable reduction of the absorption spectral peak height except in the lowest energy one. For the calculation of this problem the Fermi golden rule was not used because the density of states cannot be defined, so direct one-by-one summation was carried out over terms which depend on discrete energy levels.