The problem of incorporating interface bids into the coordinated economic dispatch for multi-area power systems is considered. Interface bids are submitted by external market participants. They facilitate interregional power transactions and preserve the financial neutrality of independent system operators. A model of interface bids is presented and its clearing process and settlement mechanism are proposed. For each independent system operator, its revenue adequacy is established and the local performance in its operating area is analyzed. Simulation results demonstrate the effectiveness of the proposed method and verify the established properties.