Radiation-induced segregation of phosphorus has been studied in nickel and Fe-15Cr-Ni alloys, with nickel content varying from 15 to 70 atomic%, using accelerator based techniques and secondary ion mass spectrometry. Experiments in pure nickel demonstrated that phosphorus segregates by an interstitial mechanism. The similarity of segregation in nickel and Fe-15Cr-Ni alloys suggests that this conclusion also applies to the alloys. We have confirmed that manganese segregates by solute-vacancy exchanges. Phosphorus, manganese, and silicon segregation in the alloys was found to be strongly dependent on nickel content. The composition dependence was, however, similar for all the solutes, independent of mechanism. Segregation was a minimum for nickel contents about 45 atomic%. Possible explanations for this are discussed.