||This paper presents the Model Access Service (MAS) developed in the EU ISTforCEproject (IST-1999-11508). It uses the capabilities of product data technology toprovide value added services in a human-centred, web-based collaborationenvironment. Along with standard services for product model access on model andobject level, it encompasses also two advanced knowledge-based features:a Reasoning Agent and an Explanation Component.MAS is developed as a self-contained system that can be used in a variety of ways.In the ISTforCE framework, it is integrated with an external Product Data Server(PDS) and a Core Information Server (CIS) which provide the necessary infrastructureenabling the full functionality of MAS.Standard functionality for model level access is provided by using MAS as a centraltool that enables generalised data exchange capabilities to all relevant productmodels in one or more construction projects, even if they are stored on differentproduct data servers. This is done on the basis of user and account informationretrieved from the CIS. To enable the use of the product models directly, MASprovides an API for generalised RPC-based model access on object level.One of the two advanced features of MAS is the integrated Reasoning Agent whichis responsible for replying to sophisticated queries concerning a specific structuraldesign model, extending the IFC2x core model. It allows clients to use AI planningmethods remotely, to generate solution sequences that combine the computed singleSolver items. The second advanced feature incorporated in the MAS is the EngineeringOntology layer. It enables the translation of IFC data to the vocabularyand semantics familiar to end users. This unique feature of the MAS, distinguishingit from most proposed product data services to date, is augmented by an ExplanationComponent which opens many of its functions to standard Web Browsers.