Both components of a polymer blend were labeled slightly with donor (naphthalene) and acceptor (anthracene), respectively and were mixed with equal amounts of various amounts of diblock copolymer. Adding diblock copolymer as a compatibilizer would improve blend compatibility and the degree of mutual segmental penetration between both components was also increased. In contrast, adding random copolymer has very little effect. Making use of nonradiative energy transfer, we can evaluate the efficiency of a compatibilizer delicately and determine the optimum concentration of compatibilizer for a blend. (c) 2007 Wiley Periodicals, Inc.