The phase behavior of poly(styrene-b-butyl methacrylate), PS-b-PBMA, diblock copolymers was investigated by small-angle neutron scattering (SANS), neutron reflectometry (NR), and rheology. For a symmetrical P(dS-b-nBMA) diblock copolymer a lower critical order transition (LCOT) at 155 degrees C was found by SANS and rheology. Furthermore, the temperature-dependent interaction parameter was determined in the temperature regime between 110 and 145 degrees C from fits to the scattering curves in the disordered region. The interaction parameter increases with increasing temperature and shows a weak temperature dependence. The LCOT behavior thus is expected to be a large entropic contribution to chi. The interfacial width observed by NR is relatively large compared to other diblock copolymers. Different block copolymers were investigated with respect to the influence of miscibility on tensile properties. While poly(styrene-b-butyl methacrylate) diblock copolymers show significant synergistic effects on tensile properties, for poly(methyl methacrylate-b-butyl methacrylate) diblock copolymers no significant synergistic effects on tensile properties were observed due to the increased interaction parameter and the smaller interfacial width as compared to the care of PS-b-PBMA diblock copolymers.