The bulk magnetic properties of (ErxY1-x)2Fe14B and (ErxPr1-x)2Fe14B systems were studied over the temperature range 4.2-1100 K. Lattice parameters, saturation magnetizations, Curie temperatures and spin reorientation temperatures were determined. Theoretical description of the detailed magnetic behavior is presented, based on a crystal field model. The (ErxY1-x)2Fe14B compounds were all found to exhibit plane-to-axis spin reorientations similar to that observed for Er2Fe14B, with the transition temperature decreasing with increasing Y content. In contrast, the spin reorientations in the (ErxPr1-x)2Fe14B systems appear to be of the cone-to-axis type. Since higher order crystal field terms appear to be significant only in the cases of Nd3+ and Ho3+, the results are discussed in terms of a crystal field Hamiltonian involving only 2nd order terms. Using known values of the exchange field, Fe anisotropy and the ratios of the crystal field coefficients, the multi-ion cr 6.tal field problem was formulated in terms of a single adjustable parameter (B0 2(f). It is shown that 2nd order crystal field terms are capable, not only of explaining the conical anisotropy of the (ErxPr1-x)2Fe14B systems, but also the decrease in the Er moment upon passing through the spin reorientation (as has been observed for Er2Fe14B). The magnetic structure of Er1.5Pr0.5Fe14B is also predicted.