The isothermal magnetic phase diagram of GdAlO3 has been measured, and a molecular field analysis yielded the following values of the coupling constants: isotropic intersublattice coupling J1=0.722 K, isotropic intrasublattice coupling J2=0.051 K, anisotropic intersublattice coupling K1=0.0438 K, anisotropic intrasublattice coupling K2=0.036 K, and crystal field anisotropy constant L0=-0.132 K. The magnetic phase boundaries calculated with these molecular field constants agreed with experiment except for the spin-flop transition near the triple point. This transition has been studied in detail as a function of the angle between the easy axis and the applied field. A first-order phase transition is found only within a critical angle as predicted theoretically. This critical angle also varies with sample shape owing to its dramatic dependence on demagnetization. The striking differences between the isothermal and the previously reported adiabatic phase diagrams are attributed to magnetocaloric cooling. The cooling on adiabatic magnetization of an antiferromagnet in its antiferromagnetic phase is calculated within the molecular field approximation (MFA) and is found to agree with this experiment and others. A comparable cooling, not predicted by the MFA, is observed for fields applied perpendicular to the easy axis. © 1971 The American Physical Society.