This paper describes a Web-services-based system which we have developed to enable organizations to semi-automatically preserve their digital collections by dynamically discovering and invoking the most appropriate preservation service, as it is required. By periodically comparing preservation metadata for digital objects in a collection with a software version registry, potential object obsolescence can be detected and a notification message sent to the relevant agent. By making preservation software modules available as Web services and describing them semantically using a machine-processable ontology (OWL-S), the most appropriate preservation service(s) for each object can then be automatically discovered, composed and invoked by software agents (with optional human input at critical decisionmaking steps). We believe that this approach represents a significant advance towards providing a viable, cost-effective solution to the long term preservation of large-scale collections of dig...