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Flexible charging and QoS provisioning are two of the main motivations behind the IP multimedia subsystem (IMS), defined by 3GPP. We have previously proposed a call differentiation solution for 3G networks. This solution defines different grades of calls with different QoS profiles, and uses a preemptive approach to give priority to more important calls over others. This idea of multi-grade services enables the users to choose the appropriate grade of call for each of their sessions, depending on the priorities/ guarantees they require. A suitable charging model is needed for the effective charging and billing of such services. In this paper, we focus on the charging aspects of multi-grade IMS services. The IMS online and offline charging mechanisms are leveraged and extended to tackle this issue. Furthermore, a proof-of-concept prototype, showcasing different multi-grade service charging scenarios, is presented.