The magnetic properties of iron particles in mercury with diameters smaller than 500 Å are sensitive to to the adsorption of tin. The intrinsic coercive force and remanence both change on the addition of tin until an amount has been added corresponding to the adsorption of roughly one monolayer. The particle sizes calculated from these adsorption data are consistent with previous size determinations. Diffusion of tin into the iron was not significant at room temperature. The addition of tin to dispersions of cobalt particles in mercury produced changes in magnetic properties related principally to a rapid diffusion of tin into the cobalt.