In the pressure testing of catalyst-loaded reactors with nitrogen gas at elevated temperatures, it is found that certain types of zeolite catalysts are substantially degraded in activity. The catalysts concerned comprises a Group VIII noble metal in an oxidized state, dispersed on a crystalline hydrogen zeolite, e.g., Y zeolite. It has now been discovered however that if such catalysts are first prereduced with a dilute, non-combustible mixture of nitrogen and hydrogen, damage on subsequent pressure testing with nitrogen is prevented or drastically reduced. Following pressure testing, the catalyst is activated by dehydration in a stream of hydrogen at elevated temperatures and pressures.