For highly disordered n-type Al0.19Ga0.33In0.48P grown lattice matched to an undoped GaAs substrate, using far-infrared-magneto-optic generalized ellipsometry, the room-temperature free-charge-carrier parameters effective mass m*=0.12(0.01) m0, concentration N=6.7(0.2)×1017 cm−3, and mobility μ=339(15) cm2/(V s) are determined by modeling the observed magneto-optic birefringence originating from the far-infrared free-charge-carrier excitations in the Al0.19Ga0.33In0.48P layer without additional electrical measurements.