The Raman spectra of single crystal KClO3 were measured in the region above 30 cm−1 at 298 and 77°K. Single crystal infrared spectra in the internal mode region were measured by reflection at 298°K. Infrared spectra of the polycrystalline material also were measured at 77°K. Assignments for all of the crystal vibrations that are derived from correlation field splitting of the (internal) ClO3− vibrations were made from the results of the polarization measurements. Longitudinal optical mode frequencies for the ν3(Bu) and ν3(Au) components obtained from polarized (infrared) specular reflection measurements on polycrystalline KClO3 are also reported. The bands due to the Raman active lattice modes were observed in the region below 200 cm−1 and were tentatively assigned to librational or to translatory-type motions. The 2-cm−1 splitting observed for the ν1 mode of the 35Cl18O16O2 and 37Cl18O16O2 isotopes in the low-temperature Raman spectra was assigned to an orientational effect in the lattice.