ObjectiveTo evaluate systematically the fidelity of a peer-befriending intervention for people with aphasia.DesignSUpporting wellbeing through Peer-befriending (SUPERB) was a feasibility randomised controlled trial comparing usual care to usual care +peer-befriending. This paper reports on the fidelity of all intervention aspects (training and supervision of providers/befrienders; intervention visits) which was evaluated across all areas of the Behaviour Change Consortium framework.SettingCommunity.ParticipantsPeople with aphasia early poststroke and low levels of distress, randomised to the intervention arm of the trial (n=28); 10 peer-befrienders at least 1-year poststroke.InterventionPeer-befrienders were trained (4–6 hours); and received regular supervision (monthly group while actively befriending, and one-to-one as and when needed) in order to provide six 1-hour peer-befriending visits over 3 months.Main measuresMetrics included number and length of training, supervision sessions and visits. All training and supervision sessions and one (of six) visits per pair were rated against fidelity checklists and evaluated for inter-rater and intrarater reliability (Gwets AC1 agreement coefficient). Per-cent adherence to protocol was evaluated.ResultsAll peer-befrienders received 4–6 hours training over 2–3 days as intended. There were 25 group supervision sessions with a median number attended of 14 (IQR=8–18). Twenty-six participants agreed (92.8%) to the intervention and 21 (80.8%) received all six visits (median visit length 60 min). Adherence was high for training (91.7%–100%) and supervision (83%–100%) and moderate-to-high for befriending visits (66.7%–100%). Where calculable, inter-rater and intrarater reliability was high for training and supervision (Gwets AC1 >0.90) and moderate-to-high for intervention visits (Gwets AC1 0.44–1.0).ConclusionPlanning of fidelity processes at the outset of the trial and monitoring throughout was feasible and ensured good-to-high fidelity for this peer-befriending intervention. The results permit confidence in other findings from the SUPERB trial.Trial registration numberNCT02947776.