Co0.9Mn0.1Fe2O4 prepared by a sol-gel method has been studied by Mossbauer spectroscopy and x-ray diffraction. The crystal structure is found to have a cubic spinel structure with the lattice constant of a(0) = 8.384 +/- 0.005 Angstrom. The iron ions at both A (tetrahedral) and B (octahedral) sites are found to be in ferric high-spin states. Its Neel temperature T-N is found to be 850 +/- 2 K. Debye temperatures for A and B sites found to be Theta (A) = 757 +/- 5 K and Theta (B) = 282 +/- 5 K, respectively. Atomic migration from the A to the B sites starts near 400 K and increases rapidly with increasesing temperature to such a degree that 52 % of the ferric ions at the A sites have moved over to die B sites by 700 K.