Both Raman scattering and infrared reflectivity measurements have been performed to determine the optic-mode spectrum of orthorhombic KNbO3 at room temperature. Pincipal-axis phonons and oblique phonons were observed with a combination of 90°scattering, back scattering, and forward scattering. Unobserved longitudinal modes and mode couplings were derived from the reflectivity measurements. The ferroelectric modes, namely those rendered highly anisotropic as a result of successive transitions, were identified and their spatial anisotropy determined; strong interference with more isotropic modes was pronounced. Dielectric constants were derived from these measurements and the A1 polariton dispersion curve was also measured. The modes were grouped according to the cubic representations from which they are derived and compared with those of BaTiO3: In particular the modes were three time less damped. Of the original soft cubic ferroelectric mode, the component along the remaining soft direction was found at 56 cm-1 and not overdamped (Γω=1). © 1976 The American Physical Society.13731093120ARC; Australian Respiratory Council Nível Superior