Kerr rotation in Gd‐Co‐Mo alloys is due mostly to the cobalt atoms. We investigate Polar Kerr loops v s composition and temperature, and determine total magnetization (from initial loop susceptibility and domain size) and the cobalt sublattice magnetization (from the saturation Kerr rotation angle). Deviations from the characteristics Kooy and Enz loop behavior due to thin ∠100Å surface layer anomalies are first identified and removed in order to analyse the bulk film behavior.