Stress corrosion cracking (SCC) studies were conducted in two commonly used pressure vessel steels: A533 Grade B Class 1 (A533-B-1) plate and an equivalent A508 Class 2 (A508-2) forging. The purpose of these studies was to determine the response of the materials in simulated pressurized water reactor environment. Round tensile specimens, 32 mm (1.25 in.) along the gage section and 9.5 mm (0.375 in.) in diameter, were bolt-loaded to 75 to 80 percent of yield and left in the water at 93 C for 2000 hr. At the end of this time period, the specimens were taken out of the water, unloaded, and examined by scanning electron microscopy (SEM) and by energy dispersive x-ray analysis. The specimen cut from A533-B-1 plate did not develop any cracks as a result of the SCC tests. Several cracks, some of which were visible with the unaided eye, had developed in the specimen that was cut from the A508-2 forging. A hydrogen-assisted cracking model has been proposed to explain the experimental results on A508-2 forging. Produced by a cathodic reaction and aided by the stress fields, hydrogen diffuses ahead of the crack tip to the inclusion sites. This causes a preferential decohesion at the inclusion matrix interfaces and subsequent cracking along inclusion bands. The absence of stress corrosion cracking in A533-B-1 plate tested under identical experimental conditions is mainly due to fewer inclusion and carbide particles and to the more refined bainitic microstructure of this steel. This type of microstructure is less susceptible to hydrogen-assisted cracking than the mainly pearlitic microstructure found in A508-2 forging.