A commercial AISI 316-Type austenitic steel (FV555) in the solution-treated condition has been irradiated in the range 575–775°C to doses of 3 to 35 dpa in the ORNL dual beam (Ni + He) facility. Irradiation-induced cavity dislocation structure has been investigated by TEM after irradiations which encompassed helium/dpa ratios of 1:1 and 20:1. The evolution of the dislocations and cavity components were found to be closely linked and dependent upon He/dpa ratio. Low-dose structure were characterised by the initial formation of faulted dislocation loops followed by loop growth and formation of small cavities at loops and network dislocations. At higher doses the loops were often ‘petular’ in shape. Extensive nucleation of helium bubbles on loops was seen under a 20:1 He/dpa ratio. Bimodal cavity size distributions apparently consisting of helium bubble and voids were found at 575 and 675°C (30 dpa: 1 appm He/(dpa)) and at 625°C (20 appm He/dpa). The critical diameter for the apparent transition between bubbles and voids was − 6 nm. Intergranular brittleness of the samples was noted between 625 and 725°C even at low He/dpa ratios: this effect occurred when helium bubbles were visible in the boundaries and was also manifest when no bubbles could be resolved. The observations indicate a strong effect of helium on the development of microstructure and of its segregation to grain boundaries leading to subsequent embrittlement at the higher temperatures.