Librarians are currently defining their roles in research data management. With their expertise in metadata curation, they can contribute a lot to metadata quality and documentation. One example is the case of the CERN Open Data Portal [opendata.cern.ch] which was made publicly accessible in November 2014. It is the access point to data produced by the research and experiments conducted at the European Organisation of Nuclear Research. The development of the CERN Open Data Portal was a collaboration between the physicists producing the data, software developers and data librarians. As this was the first release of large-size High-Energy Physics datasets, there was no experience with the documentation of these datasets. Furthermore, a metadata standard for High-Energy Physics does not exist. This presentation describes the collaboration between the researchers and the data librarians to provide extensive metadata for the data released. It also gives insight in the tailoring of the MARC21 format to accommodate all the metadata needed ad this is the format supported by the digital library software Invenio the platform is built on. Additionally, it discusses alternative metadata formats to move to in the future that might be better suitable than MARC21.