Hydrogen-producing high-pressure electrolysers operating with 40%2potassium hydroxide solution and an applied oxygen pressure up to 30 barg have been developed. Austenitic stainless steels of type AISI316L are deemed resistant to stress corrosion cracking (SCC) in concentrated KOH solutions. However, SCC has on some occasions been observed on the oxygen side of the high-pressure electrolysers, thereby representing a safety risk in the operation. Several materials have been tested for resistance to SCC using C-ring specimens in autoclaves under conditions similar to the high-pressure electrolysers, and at temperatures up to 120 C. The tests confirmed the observed susceptibility of austenitic stainless steels to SCC in concentrated KOH solutions. Higher alloyed austenitic stainless steels also showed SCC. Duplex stainless steel and nickel based Alloy 28 showed good resistance to SCC in the given environment. Further tests are needed to define the optimum weld procedure.