Ferromagnetic Heusler Co₂FeAl_0.5Si_0.5 epitaxial thin-films have been fabricated in the L2₁ structure with saturation magnetizations over 1200 emu/cm³. Andreev reflection measurements show that the spin polarization is as high as 80% in samples sputtered on unheated MgO (100) substrates and annealed at high temperatures. However, the spin polarization is considerably smaller in samples deposited on heated substrates.