The solubility limits of Be, Ti, Mo, W, P and Sb in bcc Fe and of Al, Ti, Mo and W in fcc Co were measured by X-ray diffraction and micro-probe analysis. It has been found that the solubilities in ferromagnetic regions are much smaller than those expected from the paramagnetic regions, especially in the case where the alloying element lowers the Curie temperature effectively. These solubility data have been analyzed thermodynamically, taking into account the effect of magnetic transition on the free energy of bcc Fe and fcc Co solid solutions.