In this article, an effort is made to take advantage of the inherent semantic wealth existing within Library of Congress Subject Headings (LCSHs) in order to provide more efficient subject-based information retrieval in digital libraries. To formally express such wealth, an ontology schema is presented that is capable of modeling the semantics of LCSHs into adequate ontologies. Finally, in order to show the way towards exploiting such ontologies in favor of the development of more effective subject-based information retrieval in digital libraries, a prototype system is presented. The system contains an ontology modeling the LCSHs that are employed within a digital library of theses and dissertations. Serving as a proof of concept, the prototype describes a system capable of making the expressiveness of the underlying ontology readily available to end users while at the same time retaining simplicity and ease-of-use. © 2009 Ioannis Papadakis, Konstantinos Kyprianos, Michalis Stefanidakis and Rosa Mavropodi.