Journal Citation Reports (JCR) and its journal ranking in terms of impact factor are highly influential in research evaluation. Comparisons of impact factor are valuable only when journals are of the same subject. However, a particular JCR subject category, Information Science and Library Science (IS–LS), combines two different study fields, namely Management Information Systems (MIS) and Library and Information Science (LIS). The combination of these subjects in a single category has caused the undesirable suppression of LIS journals in annual rankings. This study used papers and citation data from 88 IS–LS journals published between 2005 and 2014 to study subfield differences between MIS and LIS and their impact factor performances over 10 years. The study further examined the subfield differences within LIS, examining the differences and performances of library science, information science, and scientometric research. The results indicate that MIS and LIS are considerably different in terms of publishing and citation characteristics, cited subjects, and author affiliations. Moreover, significant differences were observed among LIS subfields. Furthermore, the results suggested that MIS and LIS pertain to two different research communities. Stakeholders must consider this difference and allow reasonable subfield differentiation and rank adjustment when using JCR for constructive research evaluations.