In order to clarify the relation between the magnetically induced phase separation and the recording media characteristics, the thermodynamic calculations of Co-Cr-Pt-B and Co-Cr-Pt-Ta systems have been carried out by using the available binary assessment data and Miedema's semiempirical values. B is segregated to the boundary in a similar manner as Cr, which makes the boundary region paramagnetic. This result is consistent with available data that B weakens the intergranular magnetic coupling and increases the magnetic anisotropy in Co-Cr-Pt recording media. By adding Ta, the Cr content in the paramagnetic phase is also increased, reducing the intergranular magnetic coupling. However, the Ta content in the ferromagnetic phase is higher than in the paramagnetic phase, decreasing the magnetic anisotropy. Accordingly, the thermodynamic calculations successfully explain experimental magnetic data for Co-Cr-Pt-B and Co-Cr-Pt-Ta recording media.