Case studies of Grodziec and Siersza mines in the Upper Silesian Coal Basin confirm that mine water accumulating in and over-flowing from abandoned coal mines is subject to a "first flush" phenomenon. The accumulated products of sulphide oxidation are dissolved in the rising mine water and flushed out at concentrations several times those observed during mine operation. Following the first overflow, sulphate concentration and hydrogen ion activity decay exponentially. In the case of workings in Siersza, decay constants of -0.003 to -0.005 day(-1) are observed, corresponding to flushing times of 480 to 820 days, some 10-20 times the period required for the workings to flood. Quantities of leachable sulphur in the abandoned workings of 0.02-0.03% are adequate to explain the observed concentrations of sulphate in the first flush, and this figure is tentatively supported by laboratory analyses.