Natural variance in the pH of three dilute lakes in the Flat Tops Wilderness Area, Colorado, complicates the detection of acidification. Variations in pH during July-September of 1983 were: 0.95 (Ned Wilson Lake), 1.36 (Upper Island Lake), and 1.53 (Oyster Lake). Mean diurnal variations in pH during 1983 were: 0.37 (Ned Wilson Lake), 0.54 (Upper Island Lake), and 0.39 (Oyster Lake). Replicate pH measurements indicate that pH can be measured with a mean variance due to measurement error of ?? 0.005. Regression analysis indicates that samples collected on the same day of different years may differ because of time of day and percentage of cloud cover. Differences in wind duration and intensity and primary productivity also may cause the pH to differ between years. Such differences can be either random or systematic. Comparisons of pH among 3 yr of data from Ned Wilson Lake indicate that natural variations in pH are much larger than variations in Colorado Lakes previously attributed to acidification by precipitation.