Laboratory experiments were conducted to determine the effect of exposure environment on the integrity of pozzolan-amended Class H cement under geologic acid gas co-sequestration conditions. Cement was exposed to two potential subsurface storage environments: (1) a supercritical mixture of CO 2 /H 2 S and (2) a CO 2 /H 2 S saturated brine. Results show that cement alteration is dependent on the amount of pozzolan addition. The alteration rate and mechanism also differ for the two exposure scenarios. Cement containing 35% pozzolan amounts by volume (hereafter referred to as 35vol% pozzolan cement) exposed to the aqueous environment was more vulnerable to alteration, compared with the same pozzolan content cement exposed to a supercritical mixture of CO 2 /H 2 S (21mol% H 2 S). Specifically, 35vol% pozzolan cement exposed to the aqueous environment exhibited a higher level of mineral dissolution (e.g., C-S-H, 3CaO·SiO 2 , 2CaO·SiO 2 , etc.) and a higher degree of sulfur alteration than the same pozzolan content samples exposed to the supercritical mixture of CO 2 –H 2 S (21mol% H 2 S). Different from the 35vol% pozzolan cement, the cement containing 65% pozzolan by volume (hereafter referred to as 65vol% pozzolan cement) was more susceptible to alteration in the supercritical CO 2 /H 2 S (21mol% H 2 S) environment, compared with the same pozzolan content cement exposed to the aqueous environment. Increasing the H 2 S mol% in the supercritical phase from 21mol% to 40mol% increased the alteration of cement exposed to both exposure environments. The 65vol% pozzolan cement was more resistant to H 2 S and CO 2 alteration than the 35vol% pozzolan cement if the H 2 S content was high (i.e., 40mol%), while the 35vol% pozzolan cement was more resistant to H 2 S and CO 2 alteration than the 65vol% pozzolan cement if the H 2 S content was relatively low (i.e., 21mol%). It was observed that while the two related exposure environments resulted in different degrees of cement alteration, the exposure environments did not result in the formation of different minerals.