Iron oxidation and hydrolysis were examined in net alkaline mine waters exiting abandoned underground lead and zinc mines. It was hypothesized that degassing of excess CO2 would result in increased pH, and thus positively influence rates of iron oxidation, despite subsequent proton production during hydrolysis. It was also hypothesized that the addition of iron oxide solids would positively influence iron removal rates. The relative roles of CO2 degassing and iron solids additions were evaluated in a field microcosm experiment in June 2001. Five treatments were established in triplicate (closed, open, open + Fe, open + aeration, open + aeration + Fe). After 36 hours, greatest water quality changes were found in the open + aeration + Fe treatment. Dissolved oxygen was greater than 7.5 mg/L and pH was greater than 7.7, while alkalinity and Fe decreased to 52 and 0.81 mg/L, respectively. In non-aerated treatments, dissolved oxygen was less than 1.6 mg/L and pH was less than 6.2. Alkalinity decreased in all aerated treatments to less than 70 mg/L, but remained greater than 200 mg/L in all other treatments. Iron concentrations remained greater than 70 mg/L in non-aerated treatments. Aeration and the active degassing of excess CO2, coupled with the presence of iron oxide solids, resulted in the greatest changes in water quality.