The magnetic phase diagram of Ho2O2SO4 (Bext ∥ a) was derived from magnetization and susceptibility measurements. Below TN = (3.43 ± 0.03) K, Ho2O2SO4 orders antiferromagnetically in a canted structure of the magnetic moments. In external magnetic fields along the a- and c-axis intermediate ferrimagnetic phases are found. At low temperatures the magnetizations of these phases are 1 3 of the respective saturation values. © 1979.