The adsorption of a nonionic surfactant on a hydrophilic surface has been treated as a phase transition between a dilute, gaslike phase (I) and a bilayer phase (II). A Kelvin-like expression for the influence of the pore curvature on the adsorption behavior could be derived. According to this expression the shift : in the chemical potential due to the curvature is proportional to the curvature energy of the bilayer. Using a molecular model, which contains a mean-field approximation, the curvature energy has been related to the affinity for the surface of the bilayer. The curvature energy at the point of phase transition increases with the affinity of the surfactants for the surface. As a consequence, the effect of curvature on the phase transition increases with affinity. The higher the affinity, the more formation of a bilayer in the pore is promoted over the same at a flat surface. The chosen phase transition model will hold as long as the adsorption changes rapidly at a given concentration, which is at least the case for surfactants with short headgroups.