The notation response of coals containing pyrite has been observed to vary with time over a period of months. This behaviour could not be attributed to coal oxidation in the normal sense. The objective of the present work was to examine whether the oxidation products of the contained pyrite were contributing to observed depression of the coal flotation response. This was achieved by examining the influence of iron sulphide oxidation products, iron(II), iron(III), sulphate and hydrogen ions on the flotation response of a number of coals. From the results it was concluded that withcleancoal, i.e., coal with little or no liberated clays associated with it, that pyrite oxidation products had little effect. However withrawcoals, i.e., flotation feeds with significant associated clays, the flotation response was markedly influenced by pyrite oxidation products particularly hydrogen ions. The effect was attributed to the presence of oleophilic organic material adsorbed on the surface of the clay.