Introduction: E-Exercise is an effective 12-week blended intervention consisting of around five face-to-face physiotherapy sessions and a web-based application for patients with hip/knee osteoarthritis. In order to facilitate effective implementation of e-Exercise, this study aims to identify physiotherapists' experiences and determinants related to the usage of e-Exercise. Methods: An explanatory sequential mixed methods design embedded in a randomized controlled trial comparing e-Exercise with usual physiotherapy in patients with hip/knee osteoarthritis. Usage of e-Exercise was based on recruitment rates of 123 physiotherapists allocated to e-Exercise and objective web-based application usage data. Experiences and determinants related to e-Exercise usage were investigated with a questionnaire and clarified with semi-structured interviews. Results: Of the 123 physiotherapists allocated to e-Exercise, 54 recruited more than one eligible patient, of whom 10 physiotherapists continued using e-Exercise after the study period. Physiotherapists had mixed experiences with e-Exercise. Determinants related to intervention usage were appropriateness, added value, time, workload, professional autonomy, environmental factors, and financial consequences. Physiotherapists recommended to improve the ability to tailor e-Exercise to the individual needs of the patient patients' individual needs. Discussion: Determinants related to the usage of e-Exercise provided valuable information for the implementation of e-Exercise on broader scale. Most importantly, the flexibility of e-Exercise needs to be improved. Next, there is a need for education on how to integrate an online program within physiotherapy.