Context-awareness is starting to emerge as a key driving principle for the design and provisioning of pervasive services in pervasive computing environments. Semantic languages seem to provide effective means to express, process and reason about context information and to facilitate knowledge sharing and interoperability among previously unknown entities. However, the exploitation of semantic languages for the design and deployment of context-aware applications requires to address several issues. In particular, semantic languages require complex and heavyweight support facilities, e.g., reasoning engines, ontology repositories and knowledge management tools, that may not fit the capabilities of portable devices, especially of resource-limited ones. Novel middleware-level solutions are required to transparently and dynamically adapt semantic-based service provisioning to the properties of different access devices. The paper proposes a novel middleware that exploits the visibility of two kinds of metadata, user/device/service profiles and policies, to tailor the configuration of the semantic support functionalities needed to access semantic-based services, and that offers a wide set of mechanisms for making viable semantic-based service provisioning to resource-constrained portable devices.