BACKGROUND: Evidence-based decision making relies on easy access to trustworthy research results. The Cochrane Library is a key source of evidence about the effect of interventions and aims to "promote the accessibility of systematic reviews to anyone wanting to make a decision about health care". We explored how health professionals found, used and experienced The Library, looking at facets of user experience including findability, usability, usefulness, credibility, desirability and value. METHODS: We carried out 32 one-hour usability tests on participants from Norway and the UK. Participants both browsed freely and attempted to perform individually tailored tasks while "thinking aloud". Sessions were recorded and viewed in real time by researchers. Transcriptions and videos were reviewed by one researcher and one designer. Findings reported here reflect issues receiving a high degree of saturation and that we judge to be critical to the user experience of evidence-based web sites, based on principles for usability heuristics, web guidelines and evidence-based practice. RESULTS: Participants had much difficulty locating both the site and its contents. Non-native English speakers were at an extra disadvantage when retrieving relevant documents despite high levels of English-language skills. Many participants displayed feelings of ineptitude, alienation and frustration. Some made serious mistakes in correctly distinguishing between different information types, for instance reviews, review protocols, and individual studies. Although most expressed a high regard for the site's credibility, some later displayed a mistrust of the independence of the information. Others were overconfident, thinking everything on The Cochrane Library site shared the same level of quality approval. CONCLUSION: Paradoxically, The Cochrane Library, established to support easy access to research evidence, has its own problems of accessibility. Health professionals' experiences of this and other evidence-based online resources can be improved by applying existing principles for web usability, prioritizing the development of simple search functionality, emitting "researcher" jargon, consistent marking of site ownership, and clear signposting of different document types and different content quality.