Addressing the obesity epidemic requires innovative, scalable behaviour change approaches with broad reach and sustainability. Mobile technology may help meet these objectives. This study aimed to develop and pilot-test a theory-based mobile telephone weight loss ‘app’, Empower, among adults. Empower was designed based on evidence-based behaviour change strategies, including control theory constructs such as setting weight loss, physical activity and dietary goals; self-monitoring; and receiving tailored feedback. A convenience sample of 16 participants, mean age 35y (SD=9), were provided with the app for their iphone (or on a loaned ipod) and asked to use it for one month. They completed pre- and post-test surveys assessing experience with mobile apps, and (post-test only) Empower app use/perceptions. At pre-test all participants reported that they had used mobile apps at least occasionally. At post-test, ten participants reported using the Empower app daily or on most days. All but two reported that they found it easy to use; all but three agreed using the app was enjoyable. However, six participants indicated that the app didn’t work the way they wanted it to, and nine noted that the app did not do everything they expected it to. Suggested improvements related to increasing flexibility to enter data, adding more goals and tips, and improving aesthetics. This study provided useful data to inform the refinement of an evidence-based weight loss app that, if shown to be effective in a future trial, could potentially help reduce the burden of obesity-related morbidity.