While problems such as small screens and inconvenient keypads have been pointed out by researchers (e.g., Thornton & Houser, 2002), we still have little knowledge of how the mobile platform affects the way in which activities are completed and how learners make decisions about using mobile phones. Stockwell (2007b) provided preliminary evidence that learners generally require more time to complete vocabulary activities and achieved slightly lower scores on mobile phones when compared to completing the same activities on desktop computers, but data in the study were limited. The current study examines 175 pre-intermediate learners of English who could choose to complete vocabulary activities on either a mobile phone or a desktop computer to identify the effect of the mobile platform. Data were collected from three cohorts of learners over a three-year period, and learner activity was analysed for the amount of time required to complete activities on both platforms and the scores they achieved for the activities. The results of the study are discussed in terms of how the platform affects learners' ability to complete tasks, whether continued usage contributes to improved performance or sustained usage of the mobile platform over time. Trends across the yearly cohorts were also identified. (Contains 4 figures and 3 tables.)