Introduction. Studies have been conducted on serendipitous encounters in the physical library environment. In this study, we aimed instead to look at features in the online library catalog that can lead to serendipitous en-counters. The aim was also to see if libraries took active steps to promote serendipity in their online library cata-logs. Method. A heuristic evaluation of four online library catalogs was conducted. The set of heuristics used was based on previous research, with the aim of identifying facets of a serendipitous environment. Semi-structured interviews were also conducted with librarians from three of the researched libraries. The results from the heuristic evaluation and the transcripts of the interviews formed the data for the analysis. Analysis. Thematic analyses were carried out on the data. The data from the heuristic evaluation and the interviews were analyzed and presented separately, leading up to a joint summary. Results. Most of the online library catalogs had features like "Similar items" and book recommendations, which can lead to serendipitous encounters. Librarians often experience a lack of influence over their online library catalogs. Librarians identified problems such as patrons expecting their online library catalog to function more like a web search engine, for example, Google Search. Conclusion. The online library catalogs have features that promote serendipitous encounters but we identified problems in how they are best designed. Using an integrated interface for the online library catalog and the library's website was one way to give the librarians more influence over their interface, which increased their likelihood to provide it with more information and content. We suggest further research with an extensive approach of serendi-pitous encounters not only in the online library catalog but also in other digital library environments such as web-sites, social media, blogs and apps. This is a two years master’s thesis in Library and Information Science.