In Workflow Management Systems (WFMSs), transaction isolation is managed most of the time by the underlying database system using ANSI SQL strategies. These strategies do not take sufficiently into account process aspects. Our work consists in studying with more depth the relation between isolation strategy and process dimension as well as the real isolation needs in workflow environments. To carry out these needs, we define `spheres of isolation' inspired from `spheres of control' proposed by C. T. Davies. Spheres of isolation take into account real workflow isolation needs with separation of concerns between workflow design and the specification of its transactional properties.