The ordering in thin films of symmetric diblock copolymers has been studied by neutron reflectivity as a function of temperature and film thickness. Exponentially damped oscillatory variations in the concentration are shown to propagate into the specimens from both surfaces with decay lengths increasing with decreasing temperature. No distinguishable order-to-disorder transition is observed; only a transition from a partially to fully ordered state is found. The temperature of this transition is found to depend in a power-law manner on the film thickness. © 1992 The American Physical Society.