The lamellar structure of a symmetric diblock copolymer was studied as a function of temperature. We used dPEP-PDMS with a molecular weight of 8.3 kg/mol as model system. The polymer was dissolved in chloroform and spin-casted on silicon wafers into thin uniform films. The degree and direction of lamellar ordering in the thin films was measured by small angle neutron scattering. Neutron reflectivity was used to measure the detailed lamellar structure in the thin films. For the temperature scans only the first order Bragg peak was measured to determine the lamellar period in the film. Our experimental results are compared to theoretical predictions on the scaling behavior of the lamella period as a function of temperature. The morphology of the surface was investigated by atomic force microscopy. Holes were found around defects in the films. The cross-section of the holes revealed the lamellar structure with a periodicity comparable to what was found by neutron reflectivity. (C) 1999 Elsevier Science B.V. All rights reserved.