We present a far-infrared (fir) spectroscopic ellipsometry study of the phonon properties of partially CUPtB-ordered (AlxGa1-x)(0.52)In0.48P with x=0, 0.32, 0.7 and 1.0, and degrees of ordering eta from 0 to 0.63, as obtained by generalized ellipsometry measurement of the near-band-gap order birefringence. An anharmonic oscillator approach is used to model the ordinary and extraordinary dielectric functions and determine the infrared-active longitudinal and transverse phonon modes of the thin-film samples. Besides the isotropic GaP-, InP- and AIP-like phonon modes, we observe alloy-induced modes with low polarity. The phonon modes of highly ordered (AlxGa1-x)(0.52)In0.48P with eta similar to 0.6 are compared to phonon modes observed in the highly disordered quaternary solid solution. We propose measurement of the fir dielectric anisotropy as a sensitive indicator for existence of sublattice ordering in multicomponent group-III-group-V semiconductor alloys. (C) 2003 Elsevier B.V. All rights reserved.