The adsorption of n-butylamine, n-butyric acid, n-butanol and 1.2 epoxy propane from n-heptane solutions onto carbon fibres (PAN) has been investigated by flow microcalorimetry. No adsorption of n-butyric acid could be detected, but the adsorption of n-butanol and 1.2 epoxy propane was reversible and relatively unaffected by surface treatments. In contrast, the adsorption of n-butylamine was very strong, partially irreversible in character and was increased markedly by surface treatment. The areas of the exothermic peaks of irreversibly and reversibly adsorbed n-butylamine were measured, and the former is considered to be a semi-quantitative estimate of the surface acidity. The surface acidity, estimated in this way, is increased by the manufacturer's surface treatment, but the external area of the fibres is relatively unchanged suggesting an increase in the fraction of acid groups at the surface. HM fibres oxidised in nitric acid show an increase in both peak areas and in the external area. When the peak areas are related to the active surface areas of the fibres, however, it is clear that the fraction of the active, or edge, surface occupied by acid groups is greater on fibres oxidised in nitric acid than on those surface treated by the supplier.