Dielectric function spectra and phonon modes of highly disordered (Al x Ga 1Ϫx ) 0.48 In 0.52 P, i.e., solid solutions with nearly randomly distributed cations, lattice matched to GaAs, are studied for Al compositions x ϭ0, 0.33, 0.48, 0.7, 0.82, and 1 using far-infrared ellipsometry and Raman scattering. An anharmonic oscillator model approach is employed for line-shape analysis of the (Al x Ga 1Ϫx ) 0.48 In 0.52 P dielectric function. A complex phonon mode behavior for the random-alloy solid solutions is found: ͑i͒ two ͑one weak GaP-like and one strong InP-like or one weak InP-like and one strong AlP-like with TO-LO splitting͒ bands are present in Ga 0.52 In 0.48 P and Al 0.52 In 0.48 P, respectively, ͑ii͒ three ͑one weak GaP-like, one weak AlP-like, and one strong InP-like͒ bands dominate the quaternary compounds for xϽ0.5, ͑iii͒ the GaP-like band is absent for xϾ0.5, and ͑iv͒ three additional modes ͑AM's͒ with low polarity occur with small composition dependencies at AM 1 ϳ313 cm Ϫ1 , AM 2 ϳ351 cm Ϫ1 , and AM 3 ϳ390-405 cm Ϫ1 , respectively. Results from polarized Raman measurements agree excellently with the mode scheme developed from the ellipsometry study. Modes AM 1 and AM 2 coincide with CuPt-type superlattice-ordering-induced lattice modes, predicted recently for Ga 0.52 In 0.48 P from first-principles calculations ͓V. Ozoliņš and A. Zunger, Phys. Rev. B 57, R9404 ͑1998͔͒ and may be used to identify small degrees of ordering in AlGaInP by far-infrared ellipsometry.