Acoustic emission activity resulting from plastic deformation of mild steel disks that were clamped and then pressurized from one side with either hydrogen or nitrogen was recorded and analyzed. It was found that during monotonic pressurization of disks in nitrogen gas, more cumulative counts were recorded than for similar disks pressurized in hydrogen gas. Possible signatures of the "births" of cracks were observed during hydrogen pressurization of disks that typically failed by leaking. The records of the nitrogen tests show very high energy and high count events occurring early in the deformation process. These events are believed to be the result of the breaking away of near-surface dislocations that had been pinned by nitrogen. The disks tested in nitrogen typically failed by bursting (ductile failure) while those tested in hydrogen typically failed by leaking ("brittle" failure). / M.S.