Cylindrical brush molecules adsorbed on a surface change their contour length and then switch conformation from rodlike to globular upon decrease of the surface energy of the substrate. The conformational changes result from partial desorption of poly(n-butyl acrylate) side chains as the surface pressure drops from 23.7 to 3.1 mN/m and the energy of interaction between the side chains and the substrate decreases from 89.7 mJ/m(2) to 69.1 mJ/m(2). At the lowest value of the interaction energy, one observes a coexistence of rodlike and globular molecules. This result is in agreement with the theoretical prediction of the rod-globule transition of surface confined brush molecules as a conformational phase transition of the first order.