A cyclic copolymer of ethylene oxide and 1,2-butylene oxide (cyclo-B(8)E(42)) was prepared by cyclization of triblock copolymer E(21)B(8)E(21), and its self-association and surface properties were studied by dynamic and static light scattering and surface tension. Critical micelle concentration (cmc), micellar molar mass and size, and molecular areas at the air-water interface were compared with corresponding quantities measured previously for linear triblock and diblock copolymers of similar chain length and composition (E(21)B(8)E(21) itself and B(8)E(41)). Under similar conditions, the cyclic block copolymer formed larger micelles than the linear triblock but smaller micelles than the. linear diblock copolymer. Cyclo-B(8)E(42) and E(21)B(8)E(21) had similar values of the cmc, about 10 times larger than that of B(8)E(41).