For interoperability between digital libraries a number of bibliographic description standards have been introduced. Some of them, like MARC21 are usually implemented in classic library systems, while new digital libraries tends to support semantically richer formats like Dublin Core or BibTeX. Although it is possible to translate back and forth between these standards, a lot of information in lost while translating from MARC21. We present MarcOnt an ontology that is based on MARC21, BibTeX and Dublin Core. We elaborate on the purpose and features of MarcOnt ontology. We describe schemata that provide underlying concepts to MarcOnt ontology. We provide an example on differences between those standards and a set of rules that are used to translate to and from MarcOnt ontology-based semantic descriptions. Finally we present the architecture of MarcOnt Mediation Service that enables cooperation.