The change in the order disorder transition temperature of block copolymer, induced by the addition of homopolymer, is investigated. Analysis of the theoretical expression based on the random phase approximation shows that the direction of the change in the transition temperature is determined by the length of the added homopolymer in relation to the length of the block copolymer and the overall composition of the monomers in the mixture. Small-angle x ray scattering measurements are performed with mixtures containing a styrene butadiene diblock copolymer and a polystyrene of various molecular weight. The observed shape of the scattered intensity curve, the angular position of the peak intensity, and the spinodal temperature for the micro-phase separation all agree well with the prediction from the theory. The values of the interaction energy density determined from the results agree fairly well with values determined previously from cloud point data. Keywords. Block copolymer, Order disorder transition, Miscibility, Polystyrene, Styrene butadiene block copolymer, Small angle X ray scattering.