## Abstract

Paramagnetic resonance in rare earth ions with an even number of unpaired 4f electrons is discussed with particular reference to experimental results in Pr$^{3+}$, Tb$^{3+}$ and Ho$^{3+}$ ions in single crystals of the ethyl sulphates. The crystal field theory of Elliott & Stevens is used to interpret the results. Some second-order perturbation mechanisms are considered which give rise to apparent quadrupole and hyperfine structure interactions. The following nuclear data are obtained from these experiments: $^{141}$Pr: I = $\frac{5}{2}$; $|\mu|$ = 3$\cdot $92 $\pm $ 0$\cdot $2 n.m.; $^{159}$Tb: I = $\frac{3}{2}$; $|\mu|$ = 1$\cdot $52 $\pm $ 0$\cdot $08 n.m.; $^{167}$Ho: I = $\frac{7}{2}$; $\mu $ = + 3$\cdot $29 $\pm $ 0$\cdot $17 n.m.; Q = + 0$\cdot $7 $\pm $ 0$\cdot $7 barm.