The asymmetric double cantilever beam fracture test has been used to study the fracture toughness of polystyrene (PS)-glass interfaces reinforced with poly(deuterostyrene-b-2 vinyl pyridine) (dPS-PVP) as a function of degree of polymerization of the blocks. The effect of modifying the glass substrate with various selfassembled monolayers is also described. For the block copolymer with degrees of polymerization, N-dPS=656 and N-PVP = 46 (referred to as N-dPS - N-PVP or 656-46), located at the interface between glass and PS, the interface fails by chain scission at areal chain densities, Sigma, of the block copolymer below a critical value, Sigma*. Above this value, e.g. Sigma > Sigma*, the interface fails by crazing followed by chain scission. For the 656-46 diblock copolymer, the transition is located at Sigma* = 0.03 chains nm(-2), which results in a calculated force to break a C-C bond along the polymer backbone of approximately 2 x 10(-9)N. For the 800-870 diblock copolymer at the interface between glass and PS, failure occurs due to chain scission. Fracture of both the 656-46 and the 800-870 block copolymers at the interface between (OTS) octadecyltrichlorosilane monolayer coated glass and PS is due to chain pulloff of the block copolymer from the OTS coated glass. Very little additional stress was transferred across the interface, resulting in fracture tough nesses com parable to that of a PS-glass interface with no block copolymer added.