The surface structure of 2-butoxyethanol (BE)-water mixtures at 300 K was studied by x-ray reflection and surface tension measurements. It is striking that a finite intrinsic surface width attributed to the surface excess BE was observed only below BE mole fraction x(BE)=0.017 where the pseudophase transition occurs in the bulk. The monolayer formation at x(BE)approximate to0.005 was found from the result of a curve fitting to the measured reflectivities by using the density profile of a single-layer "box" model in which interfaces were smeared by thermal capillary waves.