The principal magnetic moments of crystals of tri-iodohexakis(antipyrine)terbium(III)(antipyrine is 2,3-dimethyl-1-phenyl-Δ³-pyrazolin-5-one) have been measured in the temperature range 300–90 K. These are interpreted within a free-ion f⁸, ⁷F term as basis perturbed by spin–orbit coupling and a crystal field of D_3d symmetry. Values related to the coefficient of the second-order potential and for the fourth-order radial parameter [A₂⁰〈r²〉 and ρ₄, respectively] have been determined and the irrelevance of the sixth-order radial parameter, ρ₆, demonstrated. The results are compared with previous measurements down to ca. 2 K. In particular, comparison of the calculated ground-state ‘zero-field splitting’ with that previously deduced from magnetization curves demonstrates the relevance of calculations including the complete ⁷F term as basis rather than those involving only the ⁷F₆ and ⁷F₅ states, which are sufficient to account for the magnetic susceptibilities at higher temperatures.