During 24 h, water samples were taken for determination of a number of key parameters in a water system containing high concentrations of Fe^II at circumneutral pH. None of the major constituents (Ca, Mg, Na, K and sulfate) showed diurnal variations, while dissolved oxygen and pH increased during the night. This increase could entirely be explained by the decrease in water temperature. However, the concentration of Fe^II slightly increased at constant concentration of total Fe during the night, opposite to earlier observations in other systems where the presence of Fe^II was shown to be controlled by photoreduction. Nocturnal peaks of Fe^II have also been observed in other systems with high iron concentrations, however, at acidic pH, but without obvious explanation. The mechanisms for this process therefore need further investigation.