The recent advancement in network technologies has made it possible for the convergence of voice and data networks. In particular, the differentiated services (DiffServ) model has helped in bridging the diverse performance requirements between voice and non real-time applications such as file transfer and e-mail. One key feature of the DiffServ model is its promise to bring scalable service discrimination to the Internet and intranets. With the heterogeneous nature of today's distributed systems, however, there is a demand for flexible management systems that can cope with not only changes in resource needs and the perceived quality of applications (which is often user-dependent and application-specific), but also changes in the configuration of the distributed environment. Policy-based management has been proposed as a means of providing dynamic change in the behavior of applications and systems at run-time rather than through reengineering. The paper proposes a policy-enabled DiffServ architecture to provide predictable and measurable quality of service (QoS) for voice over Internet Protocol (VoIP) applications, and illustrates its performance.