Confocal microscopy (CM) and fracture reconstruction (FR), along with mechanical tests and scanning electron microscopy, were used to characterize irradiation embrittlement of a reference A533B pressure vessel steel. Toughness-temperature [K(T)] curves from tests on one-third size, pre-cracked Charpy specimens resemble the results from full sized specimens; however, the small specimen curves are shifted down in temperature and have steeper transition slopes. Cleavage fracture generally occurred by load-shedding pop-ins, arresting at distances which decreased with increasing temperature. The CM/FR measurements showed nucleation of multiple cleavage facets in high-stress regions in front of the crack tip, followed by coalescence into unstable fracture over a small increment of crack opening. At higher temperature and toughness values the damage was confined to the region around the blunting zone due to the limited specimen capacity. In this regime microvoids were also nucleated on inclusion particles. Irradiation shifted the K(T) curve about 95°C at 75 MPa.m1/2. However, no major differences in the basic cleavage fracture mechanisms were observed. Indeed at low toughness, shifts in the K(T) curve closely reflected temperature differences at a common yield stress in the irradiated versus unirradiated material. Finally, the toughness values estimated from CM/FR measurements of the critical crack tip opening displacement were in good agreement with the results of the mechanical tests.