The magnetic susceptibilities of the ions Tm3+ and Yb3+ in dilute solid solution in the cubic phase of NaYF4 were measured between 80 and 300 °K. The theory for the perturbation of the electronic states of these ions by a crystal field arising from the stereochemistry of a regular cube was able to account satisfactorily for the results. Crystal-field parameters were derived which were compared with those observed for other lanthanide ions in dilute solid solution in CaF2.