Abstract. For service providers online charging of composite services is necessary in order to manage financial risks of service delivery in multi-domain environments. At service level, inter-domain composite services consist of one or more service components, e.g. access service, IMS communication service or content service, delivered by different service providers. The 3rd Generation Partnership Project (3GPP) has developed a framework for off-line and online charging of IMS-based services. However, the current Online Charging System (OCS) specified by 3GPPP does not support an online charging function for composite services. The contribution of this work lies in the design of an online charging system that addresses the required charging functionalities based on the IMS online charging architecture. The design consists of a service composition information model guided by the NGOSS and SID concepts of the TeleManagement Forum, a set of functional charging system components and their interactions at their interfaces.