Introduction: This paper reports an exploratory study examining how users participate in social tagging activities in a scholarly digital library environment to learn about their motivations, behaviour, and practices. Method: This study was conducted in two phases: a survey to investigate usage and attitudes of social tagging tool, and a task-based user study on tagging practices in Theses@UMalaya followed by a post-task questionnaire and interview. A total of thirty -seven responses were gathered out of 160 respondents giving a 23% response rate. Analysis: Responses to the study were analysed using SPSS while qualitative data were coded to bring together the emerging themes. Results: The survey demonstrated that the majority of the participants have not used social tagging before mainly because they were unaware of the existence. The participants used keyword that most closely related to describe the documents as a tag. Usability of the social tagging application was evaluated using SUMI and PLE constructs. The participants indicated the social tagging tool was relatively more usable rather than providing them with the ability to carry out the tagging process with less effort. Conclusions: A better understanding of social tagging usage and usability will help to improve future development of social tagging tools in digital libraries. [This paper was published as part of: Proceedings of the Eighth International Conference on Conceptions of Library and Information Science, Copenhagen, Denmark, 19-22 August, 2013.]