RAMSES (Regional earth observation Application for Mediterranean Sea Emergency Surveillance) is a web-based operational thematic EO Application System for oil spill detection and monitoring. Involving many geographically distributed resources and competences, the system had to meet strong requirements in terms of platform-independency and expandability. In addition, because thematic applications using remote sensing data share a lot of commonalties, the system has been designed in a modular way to ease modules reusability. The fulfilment of these objectives was largely supported by recent technologies such as CORBA and Java. After defining the actors and services, this paper presents the architecture of the system. It then outlines further improvements that can be considered for the oil spill application itself as well as further steps required towards a full multi-application support system.