Measurements of the magnetization and differential susceptibility of GdAlO3 have been made in pulsed magnetic fields in the temperature range 1.3-100°K. It is found that GdAlO3 is a uniaxial antiferromagnet with the easy axis of magnetization along the orthorhombic b axis. The Néel temperature is found to be 3.89°K and the Curie-Weiss Θ=-4.6°K. The gradient of the inverse molar susceptibility shows the ionic moment of Gd to be 7 μB, but this full moment is never observed in the high-field magnetization, even in fields of 200 kOe at 1.3°K. The differential susceptibility has been used to observe both the antiferromagnetic-to-spin-flop and the spin-flop-to-paramagnetic phase transitions, and to determine the phase diagram in the H-T plane. The molecular-field parameters have been calculated from the antiferromagnetic-to-spin-flop transition and the angular variation of the spin-flop-to-paramagnetic transition. © 1968 The American Physical Society.