The interaction between surface-treated carbon fibres and high temperature thermoplastics (polyethersulfone and polycarbonate) was studied by measuring the interlaminar shear strength of unidirectionally reinforced composites with a fibre volume content of 60%. The ILSS-values were correlated with the surface concentration of the acidic functional groups of the carbon fibres, as analysed by temperature programmed desorption and contact angle measurements using basic aqueous solutions (pH greater-than-or-equal-to 12). Linear relationships were found with both the amount of carbon dioxide desorbed by decomposition of carboxylic surface groups and the work of adhesion calculated from the measured contact angles. This result could be interpreted as a consequence of acid-base interactions with the basic polymers. However, strongly acidic carboxylic groups are decomposed during processing of the composites. Therefore, the high ILSS-values can only be explained by chemical bonds being formed with the functional groups of the polymers during decomposition of the carboxylic surface groups.