The effectiveness of a compatibilizer in suppressing lateral phase separation in thin polyolefin blend films is investigated as a function of film thickness and temperature. Neutron and X-ray reflectivity measurements were made on spun-cast thin blend films of partially deuterated and hydrogenated polyolefin blends with and without diblock compatibilizer. We use an extended silicon surface passivating treatment. Under these biased symmetric wetting conditions (air vs hydrophobic Si), binary blend films are stabilized against both dewetting from the substrate and roughening of the surface due to phase separation when the film thickness (approximate to 25 nm) is on the order of the molecular radius of gyration. However, thicker (approximate to 100 nm) films exhibit lateral phase separation that can be suppressed by the addition of block copolymer compatibilizer. This stabilization effect can be attributed to the reduction of interfacial tension leading to a broadening of interfaces, and additionally, the presence of diblock copolymer in both phases alters their surface interactions. On longer time scales, optical micrographs show the development of large-scale features over the course of a year in the molecularly thin blend films and in the ternary thin film (stored under vacuum). A droplet morphology is observed for molecularly thin blend films, and an interconnected domain structure characteristic of the early and intermediate stages of phase separation is observed for the ternary thin blend film. The phase separated structure obtained for the binary thin blend film does not evolve with time. Our results indicate that one also needs to account for kinetics in order to develop a comprehensive understanding of the structure of thin blend films. In general, temperature has a weak effect on the profile development of compatibilized blend films.