The increased use of network communications within industries, and among firms, suppliers, and customers, is focusing greater attention on the methods and standards for interorganizational communications. In the securities indushy, the settlement and clearing of trades depends on numerous messages to be sent and received by several organizations. Using the principles of Formal Languages for Business Communication W B C ) , we develop a message representation fhat is flexible and selfdescribing, and show how defeasible reasoning applied to settlement messages could handle problem trades. This application of FLBC ofers advantages through machine-to-machine error reconcilement, integration with other market communications systems, and robustness to changes in securities design and regulation.