Large magneto-optic effects are associated with the crystal field transitions of Co++(Td) in a variety of spinel oxides. In the oxides, the F and P crystal field bands peak at about 1.5 and 0.6 μm, respectively. Many iron-containing quaternary compounds (CoTxFe2-xO4) have transition temperatures above ambient. Here, T is a trivalent metal ion such as Cr3+or Rh3+. Room-temperature magneto-optic measurements on a number of these compounds indicate peak Faraday effects in the range of 0.5 × 105to 1.0 × 105(deg/cm) at about 0.6 μm. The Curie temperature may be adjusted by the compositional parameter x and normal remanence of the surface Kerr effects are observed. The latter properties are potentially useful for optical memory devices. These materials are considered for isolation and modulation devices in the 5.0 to 12.0 μm wavelength range.