[en] Modified versions of Inconel 617, Nimonic 86 and Incoloy 800H have been manufactured. The modifications took the form of variations in the minor alloying element contents; in particular, the Mn and Ti contents and the Si and Nb contents were varied; the A1 content was kept below 0.05 wt.% in all cases. All of the modified alloys together with unmodified commercial control casts have been exposed to Oslo Phase IV helium for 2000 hours in the temperature range 800 to 1000°C. After exposure the specimens were subjected to a simple room temperature mechanical test in slow bend, to microcarbon analysis and to microstructural examination. The carburisation resistance at 800 and 900°C was moderately improved by the modifications in all three alloy bases. At 1000°C similar conclusions applied to the Incoloy 800H and Nimonic 86 bases but in the modified versions of Inconel 617 significant improvements were achieved.