We have investigated the swelling of a polystyrene brush in cyclohexane at varying temperatures with angle-dependent ellipsometry. The brush was grown in situ on the base of a high index prism by the "grafting from" technique. In the dry state it has a thickness of d = 134 nm. At the theta temperature it swells to about 600 nm. In accordance with the predictions from mean field theory, the brush thickness continuously increases with solvent quality. As the thickness increases, the shape of the segment density profile becomes smoother with a broad dilute transition region between the brush and the solvent.