The thermodynamics of mixing is shown to be quite sensitive to changes in the topology of blend components. Cyclic Bisphenol A carbonate oligomers are miscible with a wider range of polystyrene (PS) molecular weights than are chemically equivalent linear oligomers. The Flory-Huggins meanfield theory predicts the shape of phase boundaries quite well for linear PS/linear polycarbonate (PC) blends as well as for linear PS/cyclic PC blends. However, the interaction parameter is strongly dependent upon topology with chi PS1/PCC < chi PSI/PCL. This result is explained in terms of a topological repulsion between rings, which is expected to be quite general.