There is an increasing demand for polymer-bonded magnets (PBM) in high temperature applications. While most research deals with high temperature properties of NdFeB-PBM, only a few studies consider Sm-Co PBM. Therefore, this study, on the thermal and magnetic properties of Sm-Co alloy powders and blends of these with polyamide-12 (PA12), was undertaken. Since the Sm-Co powders were the product of ball milling, they contained a variety of shapes and sizes. Studies on size fractions of these showed that the thermal stability and magnetic properties were improved as the particle size increased. It was suggested that higher residual strains and smaller crystallite sizes in the small particles were responsible for a decrease in the thermal stability and magnetic properties. In addition, energy dispersive X-ray spectroscopy revealed that the oxygen content increased with decreasing particle size (larger specific surface area) and higher oxygen content was possibly also responsible for a decrease in the magnetic properties. It was shown that, in general, the surface modification by silanization, using (3-aminopropyl)trimethoxsilane, increased the saturation magnetization and remanence of both the particles and the Sm-Co/PA12 composite. The silanization also improved the thermal stability of the particles.