Abstract In this study the influence of the interface of differently treated glass fibres in epoxy resin composites is investigated by dynamic mechanical analysis. These results are compared with mechanical measurements of adhesion. It is found that the flexural storage modulus E′ of the composites with improved interfacial bonding is increased over the whole investigated temperature range. Decrease of the magnitude of tan δ at the α-relaxation associates with an improvement of interfacial bonding too. The behaviour differences can only be attributed to interfacial phenomena because all other parameters were kept constant. The dynamic mechanical analysis is an additional possibility for quantifying interfacial interactions.