Ecological momentary assessment is a method that is now largely used to study behavior and mood in the settings in which they naturally occur. It maximizes ecological validity and avoids the limitations of retrospective self-reports. Studies on the psychometric properties of scales administered via mobile phone ecological momentary assessment are lacking. Therefore, we collected data on a 4-item mood scale measuring well being on six occasions per day for 7 days (N = 307) and examined compliance rate across time, within day, and within week. Using specific latent state-trait structural equation models, we analyzed the degree to which interindividual mood differences on an occasion of measurement were because of (a) measurement error, (b) stable differences in mood level, and (c) occasion-specific differences. Results show good compliance (mean compliance: 74.9% of calls answered). Moreover, the scale showed good reliability (M = .82). Mood was mostly stable, especially the first 3 days of the week. It depended weakly albeit significantly on the previous assessment (autoregressive coefficient). In conclusion, computerized mobile phone assessment is an appropriate, easy-to-use, and promising method to measure mood.