We have used atomic force microscopy (AFM) to study the lateral structure of end grafted poly(styrene) chains on a silicon substrate in air. As the grafting density is decreased, the chains are found to ’clump’ together into islands, resulting in a highly inhomogeneous monomer density with considerable portions of the substrate exposed to air. The transition from a stable homogeneous film to the clump formation is in agreement with recent theoretical simulations of polymer brushes in different solvent conditions. The islands are found to be interconnected by thin arms, which can be visualized as isolated chains stretching out on the substrate.