The behaviour of the magnetic properties for the 3T1g term under simultaneous perturbation by spin–orbit coupling and an asymmetric (trigonal or tetragonal) ligand field is presented. The theory is applied to the variation in the magnetic properties of nineteen nickel(II) tetrahedral complexes over the temperature range 80–300°K. The magnetic moments of the complexes are related mainly to the orbital reduction factor k, and the reduction from the free-ion value of the spin–orbit coupling constant for the metal ion in the complex. The value of Δ, the axial ligand field splitting, appears to be constant and not directly related to the symmetry of the complexes.