Grid computing introduces a new paradigm for the realisation of efficient collaborative ICT infrastructures for VOs that has many advantages in comparison to other known approaches. However, due to some shortcomings with regard to industry VO requirements, practical uptake is still quite low, despite of the potential benefits. In this paper, we focus on three important issues regarding the achievement of efficient interoperability in grid-enabled VOs: (1) the appropriate handling of authorisation and authentication, (2) the role-based access to resources and services, and (3) the gridification of existing applications. We suggest an approach to extend Grid functionality by an addition- nal semantic service layer on top of the basic Grid middleware services. The services on this layer are grounded on three inter-related ontologies specified in OWL, in alignment with the Semantic Web paradigm. Reported are findings from the recently finished European project InteliGrid and the ongoing German project BauVOGrid.