We have investigated the sorption behavior of polystyrene brushes with thicknesses of 10-200 nm in toluene vapor. The sorption curve displays a kink, which is attributed to the solvent-induced glass transition. The derivative of the sorption curve with respect to solvent activity ("osmotic capacity") shows a glass step at a certain characteristic activity a, This is the analogue of the glass step observed in the heat capacity at T-g with differential scanning calorimetry. Both a, and the height of the glass step decrease with decreasing film thickness. The glass step is most pronounced after annealing the films at elevated temperatures in a vacuum. Due to accumulation of solvent in the film, the step disappears after repeated swelling/drying cycles.