Mobile technologies have not yet triggered the knowledge revolution in schools anticipated, in particular, by the telecommunications industry. On the contrary, mobile technologies remain extensively used outside the frontiers of formal education. The reasons for this are many and varied. In this paper, we concentrate on those associated with the prevalent methodological weakness in the study of innovative educational interventions with mobile technologies. In this context, the paper investigates the following question: what is the potential of second-generation cultural-historical activity theory (CHAT) for characterizing learning activities mediated by mobile technologies? To this end, an empirical study was designed with the goal of examining five small groups of students (fifth grade, age 12) who were using mobile devices in authentic educational settings, within a natural science inquiry-based learning activity outdoors. Second-generation CHAT was operationalized as an analytical and dialectic methodological framework for understanding learning activities mediated by mobile devices. The study contributes a characterization of mobile learning and identification of constraints and transformations introduced by mobile technology into students' tasks.