AbstractData are presented for the dissolved oxygen concentrations and radiometric ages of ‘normal’ (i.e. non-thermal and chemically not unusual) groundwaters in four aquifers in the Cape Province of South Africa. It is demonstrated that a significant amount of the atmospheric oxygen dissolved in the groundwater is consumed within a relatively short period of the order of 20 years. In some environments oxygen concentration measurements may provide a convenient method for locating areas of recharge in groundwater studies.