Abstract. Open source software (OSS) has been described as being designed by and for technically advanced users. As OSS has been gaining popularity among non-technical users, concern about its usability has been raised, as it is difficult for technically-minded developers to design for average users. Hiring usability experts to represent the needs of average users has been used in commercial software development as an effective solution for improving usability. It has been also suggested as a way of addressing the usability issues of OSS, but it has been observed that it is often difficult for usability experts to contribute to OSS so that their work has a major impact on the usability of the software. In this thesis, a multiple case study of four usability interventions was conducted. The cases were a part of a larger research program called UKKOSS, which aims to test ways how usability experts can meaningfully contribute to OSS by conducting usability interventions, where student teams act as usability practitioners who enter OSS projects and carry out usability work on them. This study examined how OSS developers reacted to four of those usability interventions by examining the data gathered during those interventions. The analysed data included documents, such as summary reports, communication logs, project plans, and reports on the conducted usability activities. The larger goal of studying these cases was to gather information on how usability practitioners can conduct impactful usability work on OSS projects. The outcomes of the cases were examined through the lens of prior research, and the factors that may have contributed to the success of the cases were examined through cross-case analysis. The developers welcomed the usability work of the usability teams in generally all of the four cases, but the actual impacts the interventions had varied from none of the suggested usability changes being implemented to most of them being implemented to the software. The outcomes of the most successful cases suggest that an approach where usability practitioners implement their suggested changes themselves after discussing about them with the core developers, establishing trust with the developers by contacting them via voice call or video conferencing instead of using only asynchronous communication, and making usability reports as persuasive as possible by including user testing metrics which strengthen the validity of the issues, should be studied further to evaluate if they can have a positive effect on the impact of the work of usability practitioners. The main contributions of this research were supporting the prior research on the obstacles faced by usability experts entering OSS projects by supporting it with empirical evidence and proposing new areas of research on the subject based on the outcomes of the cases
