A presentation of infidelity has been described as one of the most challenging issues to work within couple therapy. Yet the subjective experiences of therapists as they engage with couples working through infidelity remains poorly understood. We undertook a grounded theory study building on the Person of The Therapist (POTT) model to explore this phenomenon. Eight therapists with up to 20 years’ professional experience who drew on a range of professional theoretical orientations formed our sample. Our analysis led us to conclude that the concept of ambivalence best captured and described therapists’ experiences of working with infidelity. We identify three distinct facets of this theme and developed a model of its trajectory over the course of the therapeutic process. In this way, our work contributes to our understanding of how therapists deal with infidelity and provides some much-needed guidance for therapists working in this field.