Patients now turn to other patients online for health information and advice in a phenomenon known as peer-to-peer healthcare. This paper describes a model of patients' peer-to-peer engagement, based upon qualitative studies of three patient or carer groups searching for online information and advice from their health peers. We describe a three-phase process through which patients engage with peer experience (PEx). In phase I (gating) patients determine the suitability and trustworthiness of the material they encounter; in phase II (engagement) they search out information, support and/or advice from others with similar or relevant experience; and in phase III (evaluation) they make judgments about the costs and benefits of engaging with particular websites in the longer term. This model provides a useful framework for understanding web based interactions in different patient groups.