We studied phenol oxidase (PO) activity in two Sphagnum-dominated peatlands and a Carex-dominated freshwater marsh during a strong summer drought in Central New York, USA to determine whether PO activity might respond to expected climatic changes. Peat was sampled at different depths and within distinct vegetation types within the marsh. Carex-derived peat supported substantially higher PO activity (average = 0.030, range = 0.011–0.051 μMOL diqc min⁻¹ mg dry peat⁻¹, at soil pH 5.5) than Sphagnum peat (average = 0.006, range = 0.001–0.015 μMOL diqc min ⁻¹ mg dry peat⁻¹, at soil pH 3.8). In both peat types, PO activity showed a strong exponential increase with increased solution pH. Phenol oxidase activity in Sphagnum peat did not vary significantly during the drought, suggesting that PO activity may be constrained by low pH and enzyme inhibitors. Conversely, PO activity in the marsh peat varied with peat type and sample date but not as a consistent function of water-table depth. As a result, PO activity in Sphagnum peat appears to be regulated less by aeration and more by pH and possibly enzyme inhibitors. When pH is favorable, PO activity depends more on wetland vegetation type and botanical composition of the peat than climatic factors.