Abstract SANS experiments were conducted to examine the effect of block copolymer additives on the phase behavior of a binary polymer blend in the bulk state. The blend studied was a polystyrene/polybutadiene mixture, with a copolymer composed of the same homopolymers. This system exhibits upper critical solution temperature behavior. For low copolymer concentrations, the spinodal temperature decreased linearly with increasing copolymer concentration. The magnitude of the slope varies with the molecular weight of each component of the blend. As copolymer concentration increases, micelle formation is evident in the scattering function, as are the competing effects of fluctuations (macrophase separation) and ordering (microphase separation) between different types of polymers.