  Journal classification systems play an important role in bibliometric
analyses. The two most important bibliographic databases, Web of Science and
Scopus, each provide a journal classification system. However, no study has
systematically investigated the accuracy of these classification systems. To
examine and compare the accuracy of journal classification systems, we define
two criteria on the basis of direct citation relations between journals and
categories. We use Criterion I to select journals that have weak connections
with their assigned categories, and we use Criterion II to identify journals
that are not assigned to categories with which they have strong connections. If
a journal satisfies either of the two criteria, we conclude that its assignment
to categories may be questionable. Accordingly, we identify all journals with
questionable classifications in Web of Science and Scopus. Furthermore, we
perform a more in-depth analysis for the field of Library and Information
Science to assess whether our proposed criteria are appropriate and whether
they yield meaningful results. It turns out that according to our
citation-based criteria Web of Science performs significantly better than
Scopus in terms of the accuracy of its journal classification system.
