Ultrafine CoFe1.9RE0.1O4 (RE=Gd, Nd) powders have been fabricated by a sol-gel method. Magnetic and structural properties of the powders were investigated by x-ray diffractometer, Mössbauer spectroscopy, and vibrating sample magnetometer. The CoFe1.9Gd0.1O4 powders that were fired at and above 923 K contained only a single spinel phase and behaved ferrimagnetically. The grain diameters were estimated to be 11–30 nm for the Co-Gd ferrite powders fired in 773–1123 K. Mössbauer spectra measurements showed that the CoFe1.9Gd0.1O4 powders fired at 723–823 K and the CoFe1.9Nd0.1O4 powders fired at 523–1023 K had a spinel structure and were mixed paramagnetic and ferrimagnetic in nature. Mössbauer spectra of the Co–Gd ferrite powder fired at 923 K were taken at various temperatures ranging from 14 to 875 K. The iron ions at both A (tetrahedral) and B (octahedral) sites were found to be in ferric high-spin states. The Néel temperature TN was found to be 875±2 K. Debye temperatures for A and B sites were found to be ΘA=640±5 K and ΘB=217±5 K, respectively. The magnetic behaviors of the CoFe1.9Gd0.1O4 powders fi- red at and above 723 K, and CoFe1.9Nd0.1O4 powders fired at and above 923 K, respectively, showed that an increase of the firing temperature yielded a decrease in the coercivity and an increase in the saturation magnetization. The maximum coercivity and the saturation magnetization were Hc=1,149 Oe and Ms=72 emu/g in the CoFe1.9Gd0.1O4 samples and Hc=959 Oe and Ms=63 emu/g in the CoFe1.9Nd0.1O4 samples. © 2002 American Institute of Physics.