Systematic chemical changes have been observed over the well established flow cycle of Inferno Crater Lake. During recession an average 2.2 to 3.7 kg sec(-1) inflow of a diluting fluid of zero chloride and the equivalent of 423 to 660 g/t sulphate enters the lake. The euthalpy calculation indicates this is mostly composed of steam. This suggests that the sulphate is entering the lake as hydrogen sulphide which is then oxidised. The diluting fluid has a del O-18 of -5.3 to -8.4. This is consistent with the diluting fluid being either steam or steam-heated groundwater. When the lake level rises the vent fluid contains 760 to 810 g/t of chloride and 280 to 300 git of sulphate. A tentative energy balance has been calculated for the recession stage but this will need refining.