A triblock-copoly(oxybutylene/oxyethylene), denoted E58 B17 E58 (80 wt-% E), was prepared by sequential anionic copolymerisation and its micellar and gelation properties in aqueous solution were determined. The oxybutylene/oxyethylene copolymer had the advantage over comparable oxypropylene/oxyethylene copolymers of greater uniformity of composition and chain length. The methods generally used were surface tension, light scattering intensity, photon correlation spectroscopy, together with observation of gelation. Measurements were made at 30°, 40° and 55° C over a full concentration range from dilute solution into the gel (>200 g dm−3) and at specified concentrations, including the gel, over the temperature range from 10° to 70° C. The lower and upper temperature boundaries of the gel region were identified, and the occurrence of syneresis was noted. The properties investigated and discussed include critical micelle and gel concentrations, micellar weights and sizes, and structures and mobilities in the gel.