Measurements of quasistatic and dynamic switching properties of EuO have been made. EuO was pressed in the form of toroidal cores, and fired at different temperatures to obtain a variety of grain sizes. The quasistatic switching properties measured at 4°K with a 105 cps drive field (H<100 Oe) show that the hysteresis loop is not square and the remanent magnetization M r is typically 60% of saturation value. The coercivity Hc was shown to decrease with increasing grain size. For example, with a grain size increase from 55 to 220 μ, Hc decreases from 13 to 10 Oe (with H=50 Oe). Hc also decreases with decreasing drive field. The temperature dependence of Hc is similar to that of M but decreases at a slower rate for temperatures up to 63°K. The dynamic switching properties, when expressed in τ(HSingle Bond signH0) = S (τ is the switching time, H is the applied field, H0 is the threshold field, and S is the switching coefficient), of a typical sample with H c=5 Oe (at H=30 Oe) are τ(H-20) = 2.5 μsec·Oe for H≥70 Oe and τ(H-12) = 3.1 μsec·Oe for H≤70 Oe at 4.2°K for 90% switching. At low field, the flux reversal appears to be dominated by domain-wall motion; at high field the flux may reverse by nonuniform rotation. In contrast to conventional ferrite materials, EuO has slower switching speed and, consequently, has few application possibilities as computer storage elements. However, the large value of M coupled with nonsquare hysteresis loop and the insulating property make this material attractive for use in a number of low-temperature applications. © 1967 The American Institute of Physics.