The frequency distributions of mire water pH are often bimodal in data sets covering the poor-rich gradient in vegetation. Area-representative data sets of pH could indicate ecologically meaningful differences between regions, independent of differences in mire classification. Comparisons of pH results are, however, hindered by differences in water sampling methods. We conducted a field survey to assess variation of water pH related to the poor-rich gradient, the water-table-depth gradient, time of day, aeration of samples, and different methods of obtaining water samples. Considerable diurnal and fine-scale vertical and horizontal variation of pH was found in samples obtained by depressing from bryophyte capitula or directly from open water surfaces. A diurnal pattern of lower pH at night and higher at day was found, typically with a 0.5–1.0 pH unit range. Open surface-water samples showed a wide range of fine-scale horizontal variation (pH 4.6–8.0) in Scorpidium-flarks, while pipe-well samples were less variable both in unaerated samples (pH 5.0–5.3) and in aerated samples (pH 6.6–7.3). The frequency distributions of pH of both unaerated and aerated pipe-well samples were bimodal in a data set covering the transition from rich to poor fen. The higher mode was located at pH 5.6 for unaerated samples and at pH 7.6 for aerated samples. The results conformed to the hypothesis of a bimodal frequency distribution of water pH along the poor-rich gradient. A combination of measurements from unaerated and aerated water samples obtained from pipe wells is suggested as a standard method for comparable pH measurements in mires.