Symmetric poly(styrene-b-methyl methacrylate) block copolymerg (BCP) with total molecular weights of 129-412 kg/mol were blended with a symmetric 44 kg/mol BCP. This set of polymers covers the parameter space of molecular weight ratios of the large vs small BCP in the pair, R = M-n,M-L/M-n,M-S from 2.9 to 9.4. The blends macrophase separate into a nearly pure small period phase and a blended large period phase when R is greater than 5. The influence of film confinement on the phase behavior of these blends was analyzed by comparing the morphology within films on nonpreferential substrates and bulk samples using scanning electron microscopy and X-ray scatterning techniques. In the thin film, the large period phase has approximately equal volume fraction of the high and low molecular weight BCP, but in the bulk, the large period phase is more concentrated with the high molecular weight BCP. This difference in the saturation composition not only narrows the composition window for macrophase separation but also results in a 12% decrease of the large saturated period in the thin film compared to the bulk behavior of the same polymer blend. The period of the small period phase in all macrophase separating blends is comparable to that of the pure short BCP. When both phases are present, the quantity of each phase is described by the lever rule. Furthermore, when the molecular weight ratio is near the critical molecular weight ratio for macrophase separation, the blends have a transition behavior that has similarities to both the miscible and macrophase separating systems.