Mo alloying has been carried out on a Fe-Cr alloy by electron beam irradiation. It is shown that a Mo surface enrichment can be obtained in the range 2-8 at %. Scanning and transmission electron microscopy, X ray microanalysis and electron energy loss spectroscopy have been used to investigate the microstructures obtained after irradiation. Mo-Cr carbide and intermetallic chi phase have been identified after alloying. The intermetallic phase is preferentially formed when the Mo content increases. It is concluded from electrochemical studies and potentiostatic attacks that the active and transpassive dissolution rates are closely related to the Cr content in the ferritic phase. This work must be also regarded as a first step towards the surface preparation of 316 S.S. by Mo incorporation to 304 S.S.