Irradiation experiments carried out by the Comitato Nazionale Energia Nucleare to evaluate the thermal behavior of different types of fuel are described. The integral thermal conductivity has been measured continuously during the irradiation. The fuel temperatures have been detected by thermocouples, and total power has been measured by a colorimetric system able to detect separately the nuclear and gamma-ray power. The integral thermal conductivity curve versus temperature has been found for the UO2 pelletized and vibrocompacted (Vipac) powder and for the UO2-PuO2 pelletized and Sphere Pac sol-gel; the accuracy is between 10 and 12%. Differences have been found, for all materials tested, between the data at the first startup and the data measured during the following life; the differences between Vipac and pelletized samples decrease after the first startup because of the restructuring of the material.