In heap bioleaching the dissolution of gangue minerals from igneous ore materials can lead to the build-up of considerable concentrations of Mg and Al sulphates in the recycled leach solution. This may interfere with microbial ferrous iron oxidation, which drives the oxidation of the target minerals. The kinetics of the oxidation process have been well studied for Leptospirillum and Acidithiobacillus species in tank systems. Although not directly comparable, kinetic parameters derived for tank systems do apply also for heap bioleach conditions. In the present study the effect of solution concentrations of Mg and Al as sulphate at individual concentrations of 0 to 10 g/L and combined concentrations 0 to 16 g/L each has been investigated in continuous culture using Leptospirillum ferriphilum. Increasing the concentrations of the salts increasingly depresses the rate of ferrous iron oxidation and also shifts the viable range more and more into the low potential region. Al significantly reduces the amount of carbon maintained in the reactor (assumed to be commensurate with biomass), whereas Mg actually enhances it at low concentrations. In both cases, however, the rate is always depressed. The results indicate that heap cultures are likely to perform sub-optimally in those operations where build-up of dissolved gangue minerals is not controlled.