Patterns in alkalinity data for lakes and streams in Minnesota, Wisconsin, and Michigan were analyzed and mapped as a first step toward determining the potential sensitivity of surface waters to acidic deposition. The mapping was accomplished by: (1) assembling available alkalinity data on as many surface waters as possible, (2) plotting these data on relatively large-scale maps; and (3) analyzing the patterns of the 14,000 plotted values for spatial associations with other characteristics such as land use, geology, physiography, and hydrology to aid in extrapolation of the data. The level to which each factor influences alkalinity varied within the region. In northeast Minnesota and parts of upper Michigan, bedrock geology can influence surface water alkalinity. In other areas of the region, glacial drift is often very deep, and alkalinity concentrations are influenced by the composition of the drift, the lake hydrologic type, and the amount of groundwater contact. The lakes of lowest alkalinity are found in north-central Wisconsin and the upper peninsula of Michgan and tend to be small in size, comprise a small percent of total surface water area, and often occur in clusters. The mapping project helps meet the urgent need to understand the relative potential sensitivity of surface waters in the Upper Midwest, provides a tool for selecting geographic areas for more detailed studies, and allows more accurate regional assessments of effects of acidic deposition on aquatic resources.