Returning a trial questionnaire is a behaviour affected by a range of psychological and contextual factors. Previously tested Short Message Service (SMS) messages to prompt questionnaire return have not addressed these factors, and have not been characterised by established taxonomies of behaviour change techniques (BCTs). Purpose: We aimed to develop acceptable theory-based SMS messages, with fidelity to four BCTs, to support participant understanding of the consequences of not returning trial

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... questionnaires. Methods and Results: We initially developed 32 messages. Ten behaviour change experts assessed message fidelity to the intended BCT (Study 1a). All messages had appropriate fidelity to the intended BCT (mean ratings = 6.8/10 [SD = 0.6) to 7.5/10 [SD = 0.3]). Study 1b, a focus group with five patient representatives, recommended removing the BCT 'comparative imagining of future outcomes' (4 messages), two further messages be removed, and amendments to five messages. In Study 1c, 60 breast cancer survivors rated all remaining 26 messages as acceptable (mean = 3.8/5 [SD = 1.2] to 4.3/5 [SD = 0.8]). Twelve behaviour change experts rated the fidelity of the 26 messages to intended BCTs (Study 1d); all messages had appropriate fidelity (mean ratings = 6.1/10 [SD = 2.4] to 6.9/10 [SD = 1.4]). Conclusions: In these studies, we developed 26 SMS messages that were acceptable to the intended recipients and had sufficient fidelity to the intended BCTs. This approach could be taken to design interventions supporting behaviours needed for the successful delivery of clinical trials. The messages are available to research teams who can evaluate them in Studies within Trials.