Abstract Groundwaters draining Kuala Lumpur Limestone, Kinta Limestone and limestones of the Setul Formation were monitored over a 1 yr period. The closeness of the mean Mg:Ca + Mg ratios of the groundwaters (12.8%) and bedrock (13.3%) indicates calcium and magnesium dissolution to be broadly congruent. Ratios < 2.5and> 35% are much less common in groundwaters than in bedrock because of: (i) heterogeneity in bedrock mineralogy within groundwater catchments; (ii) differential solubilities of calcium and magnesium in calcites, high magnesium calcites and dolomites; and (iii) preferential precipitation of calcium in secondary carbonate deposits. In groundwaters with variable discharges, Mg:Ca + Mg correlates negatively with discharge, as calcium deposition per unit volume of water diminishes at higher flows. Use of groundwater survey data to locate areas of dolomitization and make specific inferences about the mineralogy of limestone formations is discussed, and estimates of net chemical denudation rates (range, 56.6–70.9m 3/km 2/yr) are presented.