Copper (fcc) and iron (bcc) were neutron irradiated at temperatures in the range 50 to 350 degreesC in order to study the differences in the nucleation of voids in the two metals. In Cu, void nucleation takes place only at irradiation temperatures above similar to 200 degreesC. In Fe, on the other hand, voids are formed in the whole temperature range investigated, and in addition, a high density of smaller vacancy agglomerates (micro-voids) are found to have nucleated at the lower irradiation temperatures.