Older adults are under-represented in Talking Therapies (previously named IAPT) services in the UK, a national priority for improvement in the NHS. A Talking Therapies service in the south of England identified that many older adults who were referred did not opt-in to assessment. We aimed to explore the characteristics of these older adults and understand their experiences, to inform recommendations to support them to opt-in to the service in future. First, demographic and referral characteristics were compared for older adults who did and did not opt-in, to explore any that increased odds of not opting-in. Next, surveys and semi-structured interviews were used to investigate older adults' reasons for not opting-in. Responses were thematically analysed, and themes were categorised using the COM-B model to inform theory-based recommendations. Older age, being from an ethnic minority group, having a previous referral, not being able to receive text messages, and not self-referring (e.g. being referred by GP) all significantly increased the chances of older adults not opting-in. Thematic analysis found that impersonal and confusing processes, as well as older adults' limited knowledge of Talking Therapies, beliefs about therapy, and physical, cognitive and life changes with age were barriers to opting-in. Several recommendations are made, including ideas to increase accessibility of information, change procedures to improve personal connection, and explore and overcome practical barriers. Improving routine data and feedback collection from people who do not opt-in will be important to inform and evaluate improvements.