The hydrogeochemical processes associated with the precipitation and oxidation of pyrite during the development of acid sulphate soils was investigated in the coastal floodplain environment of the Heart Morass, Victoria, Australia. During drought conditions in 2009, low-lying areas of the floodplain (0-2m elevation) were the most affected by acid sulphate soils, with a median soil pH (pH F) of 3.56 to approximately 50cm depth. Soils below ∼100cm depth in these areas contain pyrite and have reduced inorganic S concentrations of up to 0.85wt%. Higher areas of the floodplain (2-6m) do not contain acid sulphate soils, with a median pH of 4.74 to approximately 50cm depth, an average neutralising capacity of 3.87kg H 2SO 4/t, and no appreciable unoxidised pyrite. In low-lying areas concentrations of Co, Ni, Zn, Mn and Fe in soil increased from <2.0, 4.0, 10, 20 and 2000mg/kg, respectively, at 56cm depth to 10, 20, 45, 152 and 15,000mg/kg at 221cm depth. In areas of higher elevation, concentrations of Co, Ni, Zn and Fe increased from 6, 11, 21 and 12,500mg/kg at 44cm depth to 10, 19, 47 and 19,400mg/kg at 239cm depth. These data indicate acidic leaching of metals from the upper soil profile in both low-lying and more elevated areas. The lowest concentrations of Al, Co, Fe, Mn and Ni in surface water or pit water from low-lying areas were 2.43, 0.06, 2.90, 2.89 and 0.09mg/L, respectively. These concentrations are 1-2 orders of magnitude higher than in any potential water sources around the morass and are higher than can be accounted for by evapotranspiration, indicating the leaching of metals into surface water and groundwater. Excess SO42- from pyrite oxidation in the central low-lying area of the morass was characterised by molar Cl:SO 4 ratios <5 and δ 34S values <10‰. The Cl:SO 4 ratios combined with δ 34S values define zones of SO42- depletion during reduction (Cl:SO 4∼24, δ 34S=22.7‰) and contemporary SO 4 reduction of water enriched with oxidised pyritic SO42- (Cl:SO 4=9.9, δ 34S=26.2‰). Average concentrations of Fe in the upper soil profile decreased from 129g/kg during drought conditions to 15.2g/kg after flooding in 2011, suggesting the dissolution of Fe mineral salts accumulated in the upper soil profile. Average concentrations of Al, Ni, Cr and Cu increased in the upper soil profile from 9,522, 18.4, 17.0 and 14.4mg/kg during drought to 12,800, 22.4, 22.6 and 22.4mg/kg after flooding, suggesting that metal precipitation and dissolution is the result of changing pH and redox chemistry during flooding. This highlights the need for continuous measurement and sampling during flood events in order to better constrain these processes.