[en] Much of Australia's arid zone lies in a high-temperature region where potential evaporation rates can exceed the mean annual rainfall by more than an order of magnitude. Underground water therefore tends to be a biased sample of precipitation. The effect of this biasing on the stable isotopic composition and the deuterium excess of water from the Mereenie Sandstone aquifer and the western extremities of the Great Artesian Basin is discussed. An explanation is offered for an observed systematic decrease in the standard deviation of stable isotope ratios with increasing residence time. An analytical solution of a simplified version of the groundwater transport equation is presented which allows a clear mathematical distinction to be made between a 'flowing' and a 'non-flowing' aquifer system. This distinction can be useful in arid zones where the isotopic data may not be inherently predictable from the modern conditions at input because of substantial differences in recharge rates over the accessible time scales. The mathematical expressions also provide a basis for assessing when ¹⁴C data are more logically presented as absolute activities than as the conventional ¹⁴C (per cent modern carbon) ratios. (author)