The compatibility of d(8)-poly(methyl methacrylate) (d(8)-PMMA) and a series of poly(n-alkyl methacrylate)s (PnAlkMA) is studied by systematic variation of the length of the n-alkyl group from ethyl (EMA) to n-pentyl (nPenMA). Simultaneously, se series of symmetric diblock copolymers comprised of d(8)-PMMA and of PnAlkMA ranging from ethyl to n-hexyl (nHMA) is synthesized. Small-angle X-ray data of bulk samples of d(8)-PMMA-b-PEMA show that this block copolymer is in the disordered state. A change in the chi over 1/T plot occurs between 120 and 130 degrees C owing to fluctuations. All other block copolymers are in the ordered state at any temperature. The neutron reflection data of d(8)-PMMA-b-PEMA show a damped cosine concentration depth profile characteristic for thin films of block copolymers in the disordered state. In contrast, the bilayer film of the respective homopolymer pair indicates immiscibility. Neutron reflection measurements on thin films of diblock copolymers ranging from d(8)-PMMA-b-PnPrMA to d(8)-PMMA-b-PnHMA and of bilayer films of d(8)-PMMA and the respective PnAlkMAs show a decreasing apparent interfacial width with increasing length of the n-alkyl group. Taking into account the contributions of capillary waves leads to the inherent interfacial width. By use of the theoretical approach of Matsen and Bates, it is possible to extract the Flory-Huggins chi-parameter between the blocks and the average segment length.