Using time-resolved X-ray reflection experiments, we have investigated the surface structure of thin films of a symmetric diblock copolymer of polystyrene-b-polyisoprene (M(W) = 15 700). The film thickness ranges from 20 to 80 nm. The ordering transition is observed in a temperature-driven experiment. We find the transition temperature to be raised above its bulk value. It is close to the transition temperature of the ideal mean-field system. The evolution of order proceeds through a roughening of the film surface and subsequently the formation of an incomplete top layer. In atomic force microscopy measurements this layer is shown to consist of islands and depressions. The thickness of this layer is precisely one bulk lamellar period L = 16.2 nm. The lamellae are oriented parallel to the substrate. The base layer is expanded in thickness to 20 nm. The formation of the ordered structure follows the same time constants that were found for the long-range ordering process in the bulk of this polymer.