IP flow data provides input for accounting, traffic engineering and security applications. The storage of flow data is required to allow off-line processing, archiving, and sharing of traffic traces. Currently this is achieved with a variety of proprietary solutions, which results in a variety of formats as input for flow processing tools and hinders data sharing and tool reusability. In this paper we analyze the requirements for a standard file format for storing flow data, and propose using the upcoming standard IP Flow Information eXport protocol (IPFIX) as a basis for its design.