As various types of networks are proliferating with greater capacity, the demand for multimedia services, such as videoconferencing, VoIP, and IPTV, over underlying networks has been increasing. This demand has increased the need for an evaluation framework to analyze the quality of such multimedia services and network performances for the related research areas. This article proposes OEFMON, an open framework for evaluating the quality of multimedia transmissions over networks. OEFMON integrates a multimedia module and a network simulator. The major advantages of OEFMON compared to other existing work are support for any number of multimedia codecs, adaptive coding based on network feedback, on-the-fly observation of multimedia transmission, and easy extension for other tools. A couple of case studies are performed to demonstrate the applicability and usefulness of our framework.