New theoretical and experimental methods have been developed to examine the chemical and bacterial oxidation kinetics in bio-oxidation of sulphide minerals, which are determined by an 'indirect' mechanism. On-line off-gas analyses to measure the oxygen and carbon dioxide consumption rate in batch and continuous bio-reactors, together with the application of elemental balances, provides a useful method for the accurate and continuous determination of biomass and mineral specific rates, which must form the basis for reliable kinetic equations. The ferric to ferrous iron concentration ratio is a key parameter in the bacterial oxidation of ferrous iron as well as in the chemical oxidation of pyrite. In bio-oxidation experiments with pyrite extremely high [Fe3+]/[Fe2+] ratios were measured using on-line redox potential measurements. Results are illustrated for Thiobacillus ferrooxidans in continuous and batch experiments with ferrous iron, and for batch experiments with Leptospirillum bacteria with staged addition of pyrite.