Mechanical testing and surface fractography were used to characterize the fracture of E-glass fiber reinforced epoxy composites as a function of the silane coupling agent used. gamma-Aminopropyltriethoxysilane (APS) and delta-aminobutyltriethoxysilane (ABS) were used because these have been shown to have different interfacial mobilities at multilayer coverage. The values of the properties studied generally increased from untreated, ABS-, APS-treated glass-fiber reinforced composites. Strength and critical energy release rates were more sensitive to the coupling agent used, than the modulus. The flexural strengths of untreated, ABS-, and APS-treated glass-fiber reinforced composites were 449 +/-40, 510 +/- 19, and 566 +/- 9 MPa (dry state); and 389 +/- 23, 459 +/- 7, and 510 +/- 54 MPa (wet state), respectively. The critical energy release rate, G(c), as determined from a Mode I translaminar fracture toughness tests, for the untreated composites (10.5 +/- 0.4 kJ/m(2)) was lower than that for the ABS-treated composites (14.3 +/- 2.1 kJ/m(2)) which was lower than that for the APS-treated composites (17.1 +/- 2.4 kJ/m(2)). Macroscopic observations showed that a larger fiber debonding area was formed in the crack tip region for the untreated glass composites, suggesting poorer bonding compared to those treated with coupling agents. Since these silanes have similar chemistry, the differences were attributed to differences in the interfacial mobility of the coupling agent layers.