Uranium-plutonium mixed oxide sol-gel fuel is attracting interest as a promising nuclear material for fast reactor fuel subassemblies. However, this material, if employed for commercial fuel production particularly in the fast reactor area, must be extensively tested both inpile and out-of-pile. For this purpose, some irradiation tests have been carried out by CNEN in the RS-1 swimming pool reactor in Italy and in the Halden BHWR in Norway under the Plutonium Program and the Fast Reactor Program. The results obtained show: in-reactor homogenization of plutonium in the UO2 matrix, increase of the integral of conductivity after in-pile sintering process has taken place, and cladding attack caused by impurities originally present in the fuel particles.