Internet-based interventions are increasingly recognized as effective in the treatment and prevention of mental disorders. However, little research has investigated who is most likely to participate in intervention trials. This study examined the characteristics of individuals interested in participating in an online intervention to improve emotional well-being and prevent or reduce the symptoms of depression, factors reported to encourage or discourage participation, and preferences for different intervention types. The study comprised 4761 Australians participating in a survey on emotional health. Comparisons are made between those who expressed an interest in participating in the trial and those who were not. Compared to those who declined to participate, interested participants were more likely older, females, separated/divorced, and highly educated, have reported current or past history of depression, report higher depressive symptoms, and have low personal stigma. Despite the flexibility of online interventions, finding time to participate was the major barrier to engagement. Financial compensation was the most commonly suggested strategy for encouraging participation. An increased understanding of factors associated with nonparticipation may inform the design of future e-mental health intervention trials. Importantly, consideration needs to be given to the competing time pressures of potential participants, in balance with the desired study design.