First author: Taylor, J Co-author(s): Abisola Balogun1, Ben Young1, Sue Bellass1, Cath Jackson1, Mike Crooks2, David Shiers3, Richard IG Holt4, Ian Kellar5, Peter Coventry1, Jo Taylor1, Najma Siddiqi1, 2 1Department of Health Sciences, University of York 2Hull York Medical School 3Psychosis Research Unit, Greater Manchester Mental Health NHS Foundation Trust 4Faculty of Medicine, University of Southampton 5School of Psychology, University of Leeds Abstract: Rationale: People with severe mental illness (SMI) such as schizophrenia or bipolar disorder have poorer health outcomes and a shorter life expectancy by around 15 years compared with the general population. This is largely attributed to higher rates and poorer management of long-term physical health conditions (LTCs) such as diabetes. Self-management is central to successful LTC management. Although digital and non-digital interventions to support self-management are available and effective in the general population, they do not address the additional challenges faced when living with SMI. Aims: To develop and test a tailored self-management intervention that addresses the challenges people with SMI and co-existing diabetes experience in managing their illnesses. Methods: An expert consensus survey and workshop with service users, carers, healthcare staff and researchers were used to prioritise mechanisms of action (MoAs) identified from evidence about the determinants of self-management for people with SMI and co-morbid LTCs. We employed an evidence-based tool (https://theoryandtechniquetool.humanbehaviourchange.org/) to map prioritised MoAs to behaviour change techniques (BCTs), which were prioritised in a second survey in terms of their acceptability and deliverability. The prioritised BCTs were then iteratively co-designed in a series of workshops with people with SMI and diabetes, and relevant healthcare staff. Results: The expert consensus and co-design process confirmed the need for both non-digital and digital delivery of self-management support to address the multiple challenges to LTC management when living with SMI. The digital component targeted sleep, medication taking and mood through the BCTs goal setting, action planning and self-monitoring. The BCTs were found to operate through the following prioritised MOAs: Intentions; Behavioural Regulation and Memory, Attention and Decision Processes. Changing one behaviour at a time was important to service users who took part in the co-design workshops. Conclusions: In subsequent research, the feasibility, effectiveness and cost-effectiveness of the diabetes self-management intervention (DIAMONDS) will be evaluated, and learning about transferability to other LTCs will be produced. This project was funded by the National Institute for Health Research Programme for Applied Research (project number RP-PG-1016-10004).The views and opinions expressed therein are those of the authors and do not necessarily reflect those of the NHS, NIHR, or the Department of Health.