The association behaviour of a diblock copolymer of ethylene oxide and 1,2-butylene oxide, E40B10 [E denotes oxyethylene, B denotes oxybutylene, and the subscripts denote number-average block length in chain units] in aqueous solution was studied using eluent gel permeation chromatography (EGPC) over the temperature range of 9-50degreesC. In the experiment a solution of E40B10 (0.5 wt%) was used as an eluent, into which a solution of higher concentration was injected. The critical temperature for micelle formation was approximately 10degreesC. With allowance made for adsorption in the EGPC system, the extent of micellisation was determined over the full temperature range and compared with the van't Hoff prediction and with related data for triblock oxyethylene/oxypropylene copolymer F127.