Grid interoperability has recently become a major issue at Grid forums. Most of the current ideas try to solve the problem at the middleware level where unfortunately too many components (information system, broker, etc.) should be made interoperable. As an alternative concept the P-GRADE portal is the first Grid portal that tries to solve the problem at the level of workflows. It means that the components of a workflow can be executed simultaneously in several Grids. In this way the user can exploit more parallelism than inside one Grid. More than that the workflow level completely hides the low level Grid details for the end-user who does not have to learn the low level Grid commands of different Grids. In this way porting workflow application between different Grids can be done with minimal user efforts. The paper describes those features and techniques that are provided and used by the P-GRADE portal to solve the Grid interoperability problem.