Background Optimising hearing and vision function may be important in improving a range of outcomes for people living with dementia (PwD) and their companions. The SENSE-Cog cross-national randomised controlled trial (RCT) is evaluating the effectiveness of a sensory intervention (SI) to improve quality of life for PwD with concurrent hearing and/or vision impairment, in five European countries. To ascertain how or why the intervention will, or will not, achieve its outcomes, we have designed a process evaluation to explore potential discrepancies between expected and observed outcomes. This will also help us to understand how context may influence the outcomes. Here we describe the protocol for this process evaluation, which is embedded within the RCT. Methods/design We will use a mixed methods approach with a theoretical framework derived from the UK Medical Research Council’s’ guidance on process evaluations. It will include the following: (1) evaluating how key aspects of the intervention will be delivered, which will be important to scale the intervention in real world populations; (2) characterising the contextual issues, which may shape the delivery and the impact of the intervention in different countries; and (3) investigating possible causal mechanisms through analyses of potential moderators and mediators. To avoid bias, we will analyse the process data before the analysis of the main effectiveness outcomes. Discussion This evaluation will provide insight into how the complex SENSE-Cog SI will be tailored, enacted and received across the different European contexts, all of which have unique health and social care economies. The findings will provide insight into the causal mechanisms effecting change, and will determine whether we should implement the intervention, if effective, on a wider scale for PwD and concurrent sensory impairment.< Réduire