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 (PSI molecular weights than are chemically equivalent linear oligomers. The Flory-Huggins mean-field theory predicts the shape of phase boundaries quite well for linear PSAinear polycarbonate (PC) blends as well as for linear PS/cyclic PC blends. However, the interaction parameter is strongly dependent upon topology with xPSL/PC, c xPSL/PCL. This result is explained in terms of a topological repulsion between rings which is expected to be quite general.