Bio-oxidation experiments with Leptospirillum bacteria were used to determine the chemical oxidation kinetics of pyrite in acidic ferric sulphate solutions (0.1-0.2 M) at 30°C and pH 1.6. The proposed method is applicable because the oxidation of pyrite with Leptospirillum bacteria consists of two sub-processes: (i) Pyrite is chemically oxidized with ferric iron to sulphate and ferrous iron, (ii) Ferric iron is regenerated by the bacterial oxidation of ferrous iron with oxygen. The kinetics of the two sub-processes are mutually independent. It was found that the chemical oxidation rate of pyrite increases with increasing ferric to ferrous iron concentration ratios ([Fe3+]/[Fe2+]). An empirical kinetic equation appropriately described the chemical oxidation kinetics of pyrite with ferric iron. It also predicts low pyrite oxidation rates at relatively low [Fe3+]/[Fe2+] ratios (< 500), which agrees with the reported low rates under similar conditions in (sterile) chemical batch experiments. The advantage of using Leptospirillum bacteria to determine the chemical oxidation kinetics of pyrite (or other minerals) at moderate conditions is that the bacteria maintain an extremely high [Fe3+]/[Fe2+] ratio (up to 2 x 104) and at a constant level, whereas in (sterile) chemical batch experiments a steep decrease of the high initial ratio will occur in a few minutes due to the production of ferrous iron. This explains why observed bacterial oxidation rates of pyrite are a factor of 10 to 20 larger than maximum observed (sterile) chemical oxidation rates.