Digital mental health is a rapidly expanding area. Remotely delivered “e-therapies” can be an effective way for people to learn techniques to maintain their mental well-being. While e-therapies have the potential to reach many more people than face-to-face therapies can, and in a way that is convenient and potentially less stigmatizing, like other new interventions, e-therapies need to be acceptable to the people who might use them. A feasibility study, as its name suggests, asks whether the processes of an intervention are likely to work when delivered. Feasibility studies focus on such matters as intervention uptake and drop-out rates and participants’ experience of an intervention such as how easy it was to understand and use on a regular basis. This case study describes the use of a one-group feasibility study to assess a guided 2-week self-compassion intervention for mental contamination associated with negative intrusive cognitions. Mental contamination refers to a sense of inner and outer dirtiness that occurs when moral violations are recalled or imagined. This case study describes the role of a feasibility study in developing interventions and the advantages and challenges of designing and conducting a feasibility study. It explains the different methods that are used in a feasibility study and how these methods operate together to address the question of intervention acceptability.