In this study, the surfaces of activated carbon fibres (ACFs) were modified by nitric acid to introduce surface oxygen complexes and to observe the influence of those complexes on the propylamine adsorption of the ACFs. It was found that the oxygen complexes including carboxylic and phenolic groups were predominantly increased, which resulted in the increase of total surface acidity. However, the specific surface areas (^S BET) and the total pore volumes (^V t) of the modified ACFs were decreased by 5-8% due to the increased blocking (or demolition) of micropores in the presence of newly introduced complexes. Despite the decrease of textural properties, it was found that the amount of propylamine adsorbed by the modified ACFs was increased by approximately 17%. From the XPS results, it was observed that propylamine reacted with strong or weak acidic groups, such as COOH or OH, on the ACF surfaces, which resulted in the formation of pyrrolic-, pyridonic-, or pyridine-like structures.