The (100) Cu(100 A˚)‐Ni(25 A˚) superlattice structure, grown on Cu/Si(100), that shows a reversed magnetic anisotropy between the in‐plane and normal‐to‐plane magnetizations, has been heated to 400 °C, and the magnetic and structural changes investigated. The periodic structure remains mostly intact up to a 30‐min heating at 300 °C, and partially so after heating at 400 °C. The reversed anisotropy, with a hysteresis loop seen for the field perpendicular to the film plane, but not for that parallel to the film plane, remains seen up to 400 °C. This is compared with two (100) Cu/Ni/Cu structures containing 50‐ and 500‐A˚ Ni, respectively, with two 1000‐A˚ Cu layers each. The former structure, with a completely reversed magnetic anisotropy, lost both the Ni layer and the magnetic characteristics after heating at 200 °C. The latter structure retained its magnetic properties up to 300 °C, and lost both the Ni layer and the magnetic characteristics after heating at 400 °C. The different reactions involved are discussed, including that with Si and mixing between Cu and Ni.