The effect of the spin-orbit interaction of both the ground and excited configurations on the Faraday rotation of the trivalent Pr ion in praseodymium-substituted yttrium iron garnet and praseodymium trifluoride is analysed. It is found that the influence of the spin-orbit coupling strength of the ground configuration on the Faraday rotation is large; however, the MO rotation does not scale linearly with the spin-orbit coupling. It is surprising that the influence of the spin-orbit coupling strength of the excited configuration on the Faraday rotation is small except when the light frequency is near an MO resonance frequency. It is demonstrated that the spin-orbit interactions of both the ground and excited configurations are not always needed to produce a net Faraday rotation. The underlying physical reasons for the differences between our theoretical results and those obtained by other authors and between the different results obtained for Pr:YIG and , respectively, are discussed.