Mixing of near-degenerate vibrational modes by long-range dipole-dipole interactions can lead to complicated band shapes in the infrared spectra of clusters and microcrystals, particularly when the bands are inhomogeneously broadened as a result of distributions of crystal shapes and orientations in the sample. For the special case in which the clusters have isotropic crystal; structures, the equivalence of amplitude vectors for all IR-active vibrational phonon modes permits the band shapes of dipole-mixed modes to be predicted from those of isolated modes. This analysis is successfully applied to experimental infrared spectra of thin films containing weakly interacting microcrystals of CO2 or N2O, as well as to CO2 clusters prepared by condensation in the gas phase. The results prove that these gas-phase clusters of CO2 have an isotropic crystalline structure.