Abstract. Access control policies are security policies that govern access to resources. Real-time update of access control policies, that is, updating policies while they are in effect and enforcing the changes immediately, is necessary for many security-critical applications. In this paper, we consider real-time update of access control policies in a database system. We consider an environment in which different kinds of transactions execute concurrently some of which are policy update transactions. Updating policy objects while they are deployed can lead to potential security problems. We propose two algorithms that not only prevent such security problems, but also ensure serializable execution of transactions. The algorithms differ on the degree of concurrency provided.