Although the β-phase has been reported to be a supersaturated solid solution with a small amount of elements, such as iron, chromium, nickel, and so forth, it was found that the phase of Zircaloy-4 after heat treatments above 830°C quenching and 1030°C annealing is in a superparamagnetic state which contains particles of Zr(Fe_1-xCr_x)₂ so small (10 to 20 Å) that they can not be detected by usual methods. The β-phase of Zircaloy-2 may be in a similar state to Zircaloy-4 but can not be called a superparamagnetic state because of nonmagnetism of responsible precipitates. In order to give a deeper understanding of “nodular corrosion,” magnetic measurements of susceptibility and magnetization were carried out on Zircaloy-2 and -4 before and after heat treatments at 0.1 to 1 T from temperature of liquid nitrogen to room temperature. Magnetization curves at temperature of liquid nitrogen for Zircaloy-4 correspond closely to the calculated Langevin function. After heat treatments above 900°C, magnetization for Zircaloy-4 increases more than three times. This means that the amount of small particles of ferromagnetic precipitates is increased by heat treatments.