Methods and analysis The intervention comprises 12 BCTs systematically identified following systematic review and a consensus exercise undertaken with behaviour change experts. We aim to recruit n=177 participants who have experienced ACS in the previous 6 months from a local National Health Service (NHS) Teaching Hospital. Participants will be identified by Cardiac Rehabilitation staff and invited by letter to take part in the study. Those who wish to take part will be asked to access the experiment at a secure web-address and consent re-checked. Consenting participants will be randomly allocated in equal numbers to one of three study groups: i) usual care ii) usual care plus text-only BCT-based intervention or iii) usual care plus text+visual BCT-based intervention. The outcome variable will be the change in intention to phone an ambulance immediately with symptoms of ACS ≥15 minutes duration assessed using two randomised series of 8 scenarios representing varied symptoms before and after delivery of the interventions or control condition (usual care).