We present an experimental and theoretical investigation of long-wavelength optical-phonon behavior in Ga 1− x In x As alloys. We propose a model which accounts quantitatively for both phonon frequencies and Raman intensities. The transverse-optical-phonon behavior is shown to be determined by mass disorder and microscopic strains. It is shown that Raman scattering provides a means of measuring local bond distortions in mixed crystals. Coupling by the macroscopic ionic polarization is shown to determine the longitudinal optical-phonon behavior. The model accounts for the so-called anomalies of the Raman intensities and oscillator strengths observed in Ga 1− x In x As. Our analysis is supported by a thorough Raman-scattering study. The frequencies, Raman intensities and symmetries of both the transverse-and longitudinal-optical modes have been determined accurately over the whole compositional range, by using different crystal orientations ([001],[110], and [111]).