In this paper we present Mobilizar, a web-based mobile tool that facilitates the implementation and data collection of self-reported user behavior. Mobilizar was designed with both the researcher and the participant in mind. It provides investigators with a way to setup a new diary study in a matter of minutes and to electronically collect diary data from participants by using internet-enabled mobile devices. These devices promise to alleviate the burden of carrying a paper-and-pencil diary by instead using the participant’s own device. It also gives participants the flexibility to report their behavior in different ways such as making text, voice, or picture entries that fit their current situational constraints. In this paper, we describe the user interface design of Mobilizar and how it may be used to conduct diary studies with mobile devices.