We have investigated the properties of rare earth-doped InGaAsP thin films with special interest in magneto-optical device applications. Magneto-optical properties have been used in optical systems as isolators, waveguides, and switches. These materials and devices can be used to expand the functionality of InP opto-electronic integrated circuits (OEICs). Thin films of InP, InGaAs, and InGaAsP, grown by liquid phase epitaxy, were lattice matched to the (100) InP substrates. The films were n-type, with the carrier concentration decreasing by an order of magnitude in the doped films due to gettering by the rare earth elements. The doped films contained 2.6×1018 - 1.5×1020 cm−3 rare earth elements, which were observed to segregate toward the film/melt interface in the more highly doped films. A broad photoluminescence was observed at 1.52 μm in the Er-doped films. The Verdet constant was measured through the sample thickness, and the substrate signal dominated the measurements. However, the measured values were in agreement with published values for InP, which gives an indication of the films' host value. The Verdet constants increased from 4 to 7 deg/T/mm as the wavelength decreased toward the band edge. The band edges of our samples were 0.93, 1.62, and 1.30 μm, respectively. Rare earth dopants were observed to raise the refractive index of the InP films, and waveguiding at 1.3 μm was achieved in the rare earth-doped InP films and in the InGaAsP films.