<div class="htmlview paragraph">The persistent occurrence of microporosity defects in AZ91 castings has made it difficult to consistently produce sound parts. Earlier work established that dissolved hydrogen gas causes microporosity defects in AZ91; however, the exact role of hydrogen in the nucleation and growth of microporosity was not determined. In this paper, the behavoir of dissolved hydrogen gas in elemental magnesium, AZ91 alloy, and liquid binary Mg/Al alloys was studied. The results show that during the last stages of solidification, hydrogen gas is rejected from the Mg₁₇Al₁₂ intermetallic compound to assist in the nucleation and/or growth of microporosity.</div>