This paper presents a reflective architecture for process control applications with composition of metaobjects. Reflection is used to separate a model of physical entities from the monitoring and controlling part of the application and to exchange metaobjects dynamically. Dynamic exchange of metaobjects is used to implement context sensitive monitoring. Being able to program a monitoring strategy in a single metaobject, and to exchange a metaobject depending on plant state, significantly ease the programming of the monitoring and controlling part. Composition of metaobjects makes the architecture extensible and avoids to program metaobjects for all possible combinations of behaviour. A diagnosis system based on the proposed architecture has been implemented.