The evaluation of the use of library collections is a fundamental tool for the development of a relevant and cost-effective collection. Bibliometrics offers several methods to measure the level of use of collections. In this paper, we present the first results of a bibliometric analysis of the references cited in the articles published by authors of the Instituto de Astrofísica de Canarias during the last decade. We have determined which kinds of publications and which journals are mainly used. We also have studied the ages of the cited journals and their cost-effectiveness. Additionally, to see if the advent of the WWW in the mid 1990s has influenced the reading or citing practices of the authors, we have compared two sets of data: articles published over the period 1991--3 that we define as the pre-WWW period and articles published over the period 1997--9 defined as the WWW period.