The fracture toughness G(c) of an interface between the immiscible homopolymers polystyrene (PS) and poly(2-vinylpyridine) (PVP) reinforced with random copolymers, dPS(f)-r-PVP1-f was measured as a function of the average monomer fraction f and the areal chain density Sigma of the copolymer. Long symmetric random copolymers (f approximate to 0.48) are shown to be effective in strengthening the interfaces. The effectiveness of the random copolymer at low areal chain densities results from each chain establishing multiple covalent connections across the interface. Whether these connections result from each copolymer chain crossing the interface multiple times, entangling with the homopolymer on either side of the interface, or whether these connections result from a "pairing" of chains with different monomer fractions f(resulting from composition drift) is not yet certain. The interfacial G(c) increases strongly with increasing Sigma above Sigma* approximate to 0.004 chains/nm(2) where a transition from chain scission to crazing occurs. At a high areal density (Sigma > Sigma(sat), where Sigma(sat) is the saturation areal density of the copolymer, above which the random copolymer forms a distinct and continuous layer at the interface), the fracture toughness of the interface reinforced with f = 0.48 random copolymer becomes a constant. The effectiveness of the copolymer at high Sigma may be due to the presence within the random copolymers of significant fractions of chains with f > 0.48 as well as f < 0.48. Such a spread in composition is caused by composition drift during the bulk copolymerization. In a thick layer of such a copolymer at the interface, the dPS-rich chains will preferentially segregate to the PS/random copolymer interface while the PVP-rich chains will preferentially segregate to the random copolymer/PVP interface, resulting in an overall interface that is graded in composition and highly entangled. For asymmetric random copolymers (f = 0.77, 0.60, 0.39, 0.25), the effectiveness decreases markedly as the copolymer becomes less entangled with the homopolymer (corresponding to the minor component in the copolymer) on one side of the interface. The maximum G(c) for the interface saturated with the random copolymer decreases significantly as f deviates from 0.5.