While the use of mobile devices for language learning has sparked the interest of an increasing number of researchers in recent years (e.g., Aizawa & Kiernan, 2003; Thornton & Houser, 2005), our knowledge of learners' preferences for the mobile platform and their usage patterns remains limited. Are learners prepared to use mobile phones for performing language learning activities, or is there still a preference for desktop computer environments? Do learners make attempts to use mobile phones and then opt for a desktop computer instead? When and where do those learners who choose to use mobile phones use them, and why do they choose them? The current study investigated 75 learners of English at a Japanese university who were assigned vocabulary learning activities which they could choose to complete on either a mobile phone or desktop computer. It sought to determine their intentions to use mobile phones for language learning if other options were available, to compare this with their actual usage patterns, and to determine when and why learners used mobile phones. Learner attitudes and preferences were elicited through a post-survey, and usage patterns were determined through analysis of detailed server logs. The results are discussed in terms of the readiness of learners to undertake mobile-based language learning activities, and the issues having an effect on the establishment of the mobile phone as a language learning tool.