Infrared (IR) spectroscopy has been used to observe the crystal field (CF) levels of the five lowest J multiplets of Sm 3+ in regular D 4h symmetry sites in Sm 1+x Ba 2−x Cu 3 O 6+δ . From a fit to these levels, we have calculated a set of the CF parameters. Doublet features in the IR spectra have been tentatively associated with the anisotropic Sm–Cu exchange interaction. Theoretical analysis, using the superposition model, as well as density functional based ab initio calculations of the CF interaction have indicated that some of the IR bands arise from the Sm 3+ ions occupying the C 4v symmetry Ba sites. The magnetic susceptibility of single-crystalline REBa 2 Cu 3 O 6 (Re=Nd, Sm) has been measured and compared with calculations using the phenomenological CF parameters.