The effects of phosphorus on void swelling in austenitic stainless steels under fast neutron irradiation have been investigated. Three model alloys (Fe-16Cr-17Ni, Fe-16Cr-17Ni-0.024P, Fe-16Cr-17Ni-0.1P) were irradiated to 40 dpa in the Fast Flux Test Facility (FFTF) using Materials Open Test Assembly (MOTA). The irradiation temperatures were 679K, 793K and 873K. Addition of phosphorus resulted in a suppression in void swelling at all temperatures. At the higher temperatures (793K and 873K), radiation enhanced formation of M2P-type phosphides were observed. These phosphides are expected to act as strong defect sinks, causing suppression of void swelling at these temperatures. On the other hand, at the lower temperature (679K), the suppression of void swelling appeared to be related to the reduction of vacancy mobility by phosphorus addition.