Objectives: There is currently little known about why people decline to participate in dyadic, psychosocial dementia research. This interview study aims to explore the reasons why people declined to participate in the Valuing Active Life in Dementia research trial. Methods: Ten family carers of people with dementia, who were part of a dyad that had declined to take part in the randomised controlled trial, participated in qualitative telephone interviews to explore their reasons for declining. Inductive thematic analysis was used to identify themes. Findings: Two themes with related sub-themes were identified: (1) Protectiveness - protecting the person with dementia, themselves as carers and their current lifestyle; (2) 'It's not for us' - the time commitment, and the possible unsuitability of the intervention, was seen to outweigh the perceived benefit of taking part. People with dementia were not always involved in the decision-making process, with carers stating the decision not to participate was made in the usual way as all their decisions. No apparent differences between the spousal and the child carers were apparent in the small sample. Conclusion: Recruitment to randomised controlled trials can be considered difficult or unfair because some participants will miss out on the desired intervention. However, this study shows that concern about the time and inconvenience of being involved in the trial can put people off research participation. Identifying possible reasons for declining research participation contributes to the design of future trials and recruitment strategies, so that the potential benefit is considered relative to the time and effort involved. Offering research opportunities to people with dementia and their families at the right stage of the dementia trajectory for their needs, facilitating personalised recruitment strategies with finely tailored researcher communication skills should help maximise recruitment, reduce attrition and deliver a more successful trial.