Although the evaluation of knowledge in the form of a quantitation of scientific output is not uncontroversial, it is a widely practised form of science evaluation. For more than 30 years, the Science Citation Index (SCI) has been alone in fulfilling this purpose. But since 2005 Scopus is a direct competitor to the SCI-databases. Comparing the two databases should help to answer questions that could have repercussions for the future generation of bibliometric analyses. The results of the comparison will allow us to more reliably rate Scopus, as a new data source, against the established SCI. In future, people who generate bibliometric analyses must be able to justify why they chose to use one database and not the other. It will not be enough to simply claim that SCI is the established source.