A coalition consists of independent organizations that share resources and skills to achieve significant mission objectives. Dynamic Coalition (DC) formations occur in response to some market demands, business requests, or disaster responses, to name a few. Partners forming a coalition are automatically selected given some business criteria and become active participants from the time the coalition is formed. Highly dynamic coalitions (HDCs) form a sub class of dynamic coalitions where the coalition formation and operation are strictly bound by time in order to provide a prompt reaction to some events. This type of dynamism poses the necessity of underlying security models and technologies allowing for automated coalition formation and operation. This paper presents a platform-driven approach to HDCs. It first defines a life cycle inherent to HDC formations, and then presents a platform-driven access control model that takes advantage of semantics of partners' requirements to provide interoperable access control to resources shared in a coalition. Coalition partners can achieve a high level of service interoperation by enhancing their access control requirements with semantics of usage, and interlinking their semantics using class relations based on standard ontology. Copyright © 2009 John Wiley & Sons, Ltd.