The effect of acid-volatile sulfide (AVS) oxidation on Fe-S cycling and water quality in coastal flood plain drains from acid-sulfate soil landscapes was examined using natural sediments and synthetic iron monosulfide. Oxidation of AVS occurred rapidly (half-time ≤ 1 h) and produced elemental sulfur (S80(s)) and iron oxyhydroxide (FeOOH (s)). The initial rapid AVS oxidation process occurred without significant acidification or changes to the aqueous-phase composition. Severe acidification (pH < 4) occurred only once S80 (s) began to oxidize to SO4 (within 2-3 days of the initial AVS oxidation). Our results demonstrate, for the first time with natural sediments, a significant pH-buffered (near-neutral) AVS oxidation step with the trigger to acidification being the oxidation of S80 (s). Acidification resulted in the pH-dependent release of large amounts of Al, Mn, Ni, and Zn even though the sediment metal content was low.