This preliminary study examined the relationship of interfacial shear strength (IFSS) to modulus of rupture (MOR) for several different wood-plastic composites (WPCs). Plastics utilized were high-density polyethylene (HDPE) and polypropylene (PP). Pine and oak were used as the wood species. Compatibilizers used included maleic-anhydride-grafted polyethylene (MAPE) and maleic-anhydride-grafted polypropylene (MAPP). A strong correlation between IFSS and MOR was observed in a limited composition region. PP/oak WPCs showed poor correlation between MOR and IFSS, probably due to the roughness of the oak surface. Compatibilizer systems incorporating liquid components did not give good results and appeared to be inappropriate for this test method.