Oxyethylene/oxypropylene block copolymers were prepared with almost identical overall compositions (40 wt.% E) and molecular weights (Mn= 2900) but different block structures, i.e. diblock E26P29 and triblock E14P30E14. Their micellar and surface properties were studied by static and dynamic light scattering and surface tension. At a given temperature, the surface acitivity, the c.m.c., the molar micellar weight and the micellar size did not differ within the uncertainties of their determination. The only significant difference was in the area per molecule in the surface monolayer, wherein the diblock copolymer was more closely packed than the triblock copolymer.