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Tensile test results at 25 and 300°C on five 9-12Cr tempered martensitic steels and one 14Cr oxide dispersion strengthened alloy, that were side-by side irradiated to 6.5 dpa at 295°C in the Advanced Test Reactor (ATR), are reported. The engineering stress-strain curves are analyzed to provide true stressstrain constitutive ()laws for all of these alloys. In the irradiated condition, the () fall into categories of: strain softening, nearly perfectly plastic and strain hardening. A range of increases in yield stress ( y ) and reductions in uniform strain ductility (e u ) are observed, where the latter can be understood in terms of the alloy's () behavior. Increases in the average () in the range of 0-10% strain are smaller than the corresponding  y , and vary more from alloy to alloy. The data are also analyzed to establish relations between  y and coupled changes in the ultimate stresses as well as the effects of both test temperature and the unirradiated yield stress ( yu ). The latter shows that higher  yu correlates with lower  y . In five out of six cases the effects of irradiation are generally consistent with previous observations on these alloys. However, the particular heat of the 12Cr HT-9 tempered martensitic steel in this study has a much higher e u than observed for earlier heats. The reasons for this improved behavior are not understood and may be microstructural in origin. However, it is noted that the new heat of HT-9, which was procured under modern quality assurance standards, has lower interstitial nitrogen than previous heats. Notably there is a general relation between lower interstitial solute contents and improved ductility, and homogenous deformation, in broadly similar steels.