We present a far-infrared spectroscopic ellipsometry study of the phonon properties of partially CuPtB -ordered (AlxGa1-x)0.52In0.48P with x =0, 0.32, 0.7, and 1.0 and degrees of ordering η = 0 … 0.63, determined from 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 transversal phonon modes of the thin film samples. Besides the isotropic GaP-, InP- and AlP-like phonon modes, we observe alloy-induced modes with low polarity for η ∼ 0. The evolution of lattice like and alloy-induced modes with increasing η is reviewed for Ga0.52In0.48P. With increasing η all phonon modes split in E- and A1 -type vibrations with polarization perpendicular and parallel to the sublattice ordering direction, respectively, and the mode frequencies shift continuously. Our observations extend recently published phonon frequencies from experiment and theory for CuPt-ordered Ga0.52In0.48P, which are compared and discussed. The phonon modes of highly ordered (AlxGa1-x)0.52In0.48P, η ∼ 0.6, are compared to the modes observed in the highly disordered quaternary solid solution. We propose the far-infrared dielectric anisotropy as a sensitive measure for sublattice ordering in multinary III-V alloys.