Question: What are clinical physiotherapists' perceptions about delivering two interventions during a randomised trial: the MOBILISE trial? Design: Mixed methods study using semi-structured interviews involving closed- and open-ended questions. Participants: Thirteen physiotherapists involved in delivering the intervention for the trial. Results: All thirteen physiotherapists (100%) had a preference for their patients to get one of the interventions, mostly dependent on the individual patient. Most were frustrated if their patients were not allocated to their preferred intervention but 62% were satisfied with the intervention they delivered and 100% would be happy to be involved in future research. Two significant themes emerged from the open-ended data: that there were both positive and negative aspects of being involved in the trial. The positive aspects included the trial's value as a way of participating in research and as a way of providing evidence for practice. The negative aspects were that the design of the trial was not always reflective of usual clinical practice and the trial's impact on departments, therapists and patients. Conclusion: Clinical physiotherapists had both positive and negative perceptions about delivering two different interventions in a clinical trial. However, they were all interested in participating in future research, suggesting that the positive aspects outweighed the negative. [Bampton J, Vargas J, Wu R, Potts S, Lance A, Scrivener K, Ada L, Dean C (2012) Clinical physiotherapists had both positive and negative perceptions about delivering two different interventions in a clinical trial: a mixed methods study. Journal of Physiotherapy 58: 255-260].