Modern instruments in astrophysics lead to a growing amount of data and more and more specific observations, among which scientists must be able to identify and retrieve useful information for their own specific research. The Virtual Observatory ( http://www.ivoa.net/deployers/intro_to_vo_concepts.html ) architecture has been designed to achieve this goal. It allows the joint use of data taken from different instruments. Retrieving and cross-matching those data is in progress, but it’s impossible today to find a sequence resolving a given science case needing a combination of existing services of whom the user doesn’t knows the specifications. The goal of this work is to propose the basis of an architecture leading to automatic composition of workflows that implement scientific use cases.