The emerging service-oriented computing paradigm advocates building distributed information systems by chaining reusable services instead of by programming from scratch. To do so, not only business processes, but also business rules, policies and constraints need to be encoded in a process language such as Web Services Business Process Execution Language (WS-BPEL). Unfortunately, the intermixing of business processes and rules in a single process weakens the modularity and adaptability of the systems. In this paper, we propose a formal approach to model the weaving of business processes and rules, following the aspect-oriented principle. In particular, we use Predicate/Transition (PrT) nets to model business processes and business rules, and then weave them into a coherent PrT net. The resulting woven nets are ready for analysing system properties and simulating system behaviour.