In 2008, the type of document “proceedings paper” (PP) was assigned in the WoS database to journal articles which were initially presented at a conference and later adapted for publication in a journal. Since the use of two different labels (“article” and “proceedings paper”) might lead to infer differences in their relevance and/or quality, this paper presents a comparative study of standard journal articles and PP in journals to explore potential differences between them. The study focuses on the Library and Information Science field in the Web of Science database and covers the 1990-2008 period. PP approximately account for 9% of the total number of articles in this field, two-thirds of which are published in monographic issues devoted to conferences, which tend to be concentrated in specific journals. Proceedings papers emerge as an heterogeneous set comprising PP in ordinary issues, similar to standard articles in structure and impact of research; and PP in monographic issues, which seem to be less comprehensive and tend to receive less citations. Faster publication of PP in monographic than in ordinary issues may conceal differences in the review process undergone by either type of paper. The main implications of these results for authors, bibliometricians, journal editors and research evaluators are pointed out.