The microphase separation in a symmetric diblock copolymer consisting of polystyrene and polyparamethylstyrene has been studied by small-angle neutron scattering. The observed peak changes with temperature in intensity, shape and position. The peak position shifts at the microphase separation transition temperature T(MST) by approximately 7%. Also from a peak intensity analysis we determine T(MST) at approximately 180-degrees-C. The shape of the peak change at T(MST) and the indication of a second-order peak is observed, characteristic of a lamellar phase in the ordered regime. Intensity does not vanish for q --> 0, but has a temperature-dependent level. Results are discussed with respect to recent computer simulations and theoretical approaches. A simple mean-field picture is not consistent with the data.