A method of processing Accounting Request ACR messages in an IP Multimedia Subsystem, IMS network, the method comprises: for any ACR [Interim] or ACR[Stop] for a session received by a charging collection function node, if a corresponding ACR [Start] is not there with the charging collection function node, the ACR [Interim] or ACR[Stop] is stored in an incomplete database associated with the charging collection function node; said charging collection function node being able to communicate with a correlation host provided with a correlation functionality for the ACRs, a correlation database for CDRs and an incomplete database for the ACRs; if the correlation host is in-service, the charging collection function node processes the ACRs as follows: if the charging collection function node is sending aggregated CDRs, then those CDRs are written into the correlation database (92; 92-1) of the correlation host; if the charging collection function node is sending ACRs from the incomplete database, then those ACRs are written into the incomplete database of the correlation host; for each session in the charging collection function node and the correlation host's incomplete databases, the following applies: the charging collection function node in question is the correlation host for the session, but it is missing ACR [Start] and/or ACR [Stop] information for the session; the charging collection function node in question is not the correlation host, but it cannot send these records to the correct correlation host since that host is out of service. for any ACR [Interim] or ACR[Stop] for a session received by a charging collection function node, if a corresponding ACR [Start] is not there with the charging collection function node, the ACR [Interim] or ACR[Stop] is stored in an incomplete database associated with the charging collection function node; said charging collection function node being able to communicate with a correlation host provided with a correlation functionality for the ACRs, a correlation database for CDRs and an incomplete database for the ACRs; if the correlation host is in-service, the charging collection function node processes the ACRs as follows: if the charging collection function node is sending aggregated CDRs, then those CDRs are written into the correlation database (92; 92-1) of the correlation host; if the charging collection function node is sending ACRs from the incomplete database, then those ACRs are written into the incomplete database of the correlation host; if the charging collection function node is sending aggregated CDRs, then those CDRs are written into the correlation database (92; 92-1) of the correlation host; if the charging collection function node is sending ACRs from the incomplete database, then those ACRs are written into the incomplete database of the correlation host; for each session in the charging collection function node and the correlation host's incomplete databases, the following applies: the charging collection function node in question is the correlation host for the session, but it is missing ACR [Start] and/or ACR [Stop] information for the session; the charging collection function node in question is not the correlation host, but it cannot send these records to the correct correlation host since that host is out of service. the charging collection function node in question is the correlation host for the session, but it is missing ACR [Start] and/or ACR [Stop] information for the session; the charging collection function node in question is not the correlation host, but it cannot send these records to the correct correlation host since that host is out of service.