The effects of interfacial treatment of glass fibres in glass/epoxy composites were studied through Mode I delamination fracture toughness tests using a double cantilever beam specimen. The treatment of glass fibres with two similar silane coupling agents has been shown to improve the mechanical properties of the composite as a function of the type of coupling agent. gamma-aminopropyltriethoxysilane (APS) and gamma-aminobutyltriethoxysilane (ABS) have similar chemistry, but differ in mobility (molecular motion) at the coupling agent-epoxy interface. The critical energy release rate, G(1c), for the APS-treated composites (0.59 +/- 0.05 kJ m(-2)) was shown to be higher than that of the ABS-treated one (0.37 +/- 0.01 kJ m(-2)) and also the untreated one (0.31 +/- 0.02 kJ m(-2)). In this case, the bulk structural property appears to be a function of the microscopic interfacial properties including the dynamics of the coupling agent layer. Optical characterization of the fracture surfaces reveal delamination at the epoxy-glass interface for the untreated samples, while the ABS- and APS-treated samples showed less interfacial delamination, respectively.