Random copolymer layers are surprisingly effective at reinforcing polymer-polymer interfaces. One hypothesis is that composition drift during synthesis can account for the higher than expected toughening. To test this hypothesis, we polymerized a series of poly(d-styrene-r-2-vinylpyridine) (dPS(f)-r-PVP1-f) copolymers with various fractions (f) of deuterated styrene to only 10% completion to avoid composition drift. The fracture energies (G(c)) of polystyrene/dPS-r-PVP/poly(2-vinylpyridine) interfaces with relatively thick layers of dPS-r-PVP were measured. G(c) decreased relative to interfaces reinforced with composition-drifted dPS-r-PVP. Conversely, G(c) increased when two or more copolymers were blended together. In such samples, the copolymers form distinct layers with multiple interfaces characterized by the difference in f (Af) between adjacent layers. We find that G(c) is governed by Deltaf(max), the largest difference in adjacent compositions, and, therefore, by the width of the narrowest interface (w(min)). G(c) increases strongly as w(min) increases from 3 to 5 nm. Remarkably, these w(min) values are about half the entanglement spacing in bulk polystyrene.