This paper examines non-response in a mobile app study designed to collect expenditure data. We invited 2,383 members of the nationally representative Understanding Society Innovation Panel in Great Britain to download an app to record their spending on goods and services: participants were asked to scan receipts or report spending directly in the app every day for a month. We examine participation at different stages of the process. We further use data from the prior wave of the panel to examine the prevalence of potential barriers to participation, including access, ability and willingness to use different mobile technologies, and biases in the types of people who participate, considering socio-demographic characteristics, financial position and financial behaviours. While the participation rate was low, drop out was also low: over 80% of participants remained in the study for the full month. The main barriers to participation were access to, and frequency of use of mobile devices, willingness to download an app for a survey, and general cooperativeness with the survey. While there were strong biases in who participated in terms of socio-demographic characteristics (with women, younger, and more educated sample members being more likely to participate), and in terms of financial behaviours (with respondents who already use mobile devices to monitor their finances being more likely to participate), we found no biases in correlates of spending.