BackgroundConsiderable evidence supports the efficacy of e-interventions for mental health treatment and support. However, client engagement and adherence to these interventions are less than optimal and remain poorly understood.ObjectiveThe aim of the current study was to develop and investigate the psychometric properties of the e-Therapy Attitudes and Process questionnaire (eTAP). Based on the Theory of Planned Behaviour (TPB), the eTAP was designed to measure factors related to client engagement in e-interventions for mental health.MethodsParticipants were 220 adults who reported current use of an e-intervention for mental health support. Participants completed the eTAP and related measures, with a subsample of 49 participants completing a one-week follow up assessment.ResultsA 16-item version of the eTAP produced a clear four-factor structure, explaining 70.25% of variance. The factors were consistent with the TPB, namely, Intention, Subjective Norm, Attitudes, and Perceived Behavioural Control. Internal consistency of the total and subscales was high, and adequate to good one-week test retest reliability was found. Convergent and divergent validity of the total and subscales was supported, as was the predictive validity. Specifically, eTAP Intentions correctly predicted engagement in e-interventions with 84% accuracy and non-engagement with 74% accuracy.ConclusionsThe eTAP was developed as a measure of factors related to engagement and adherence with e-interventions for mental health. Psychometric investigation supported the validity and reliability of the eTAP. The eTAP may be a valuable tool to understand, predict, and guide interventions to increase engagement and adherence to e-interventions for mental health.