English as a Foreign Language (EFL) students face a daunting challenge. They must study and memorize literally thousands of words to achieve fluency. Knowledge of vocabulary not only allows for clearer expression, but also forms the foundation of language knowledge. Mobile devices such as smartphones and tablet PCs can potentially be effective learning tools, especially when used for developing the vocabulary of EFL students. While there are many good reasons to use technology to advance vocabulary learning, there are also several disadvantages to consider. In light of this, the main purpose of this study is to determine EFL student differences in vocabulary learning based on the use of mobile devices instead of more traditional methods. A total of 120 Taiwanese senior high school and undergraduate students with a score at the intermediate level of the General English Proficiency Test (GEPT) was divided into three groups. Each group was given twenty classes over one month. A teacher, in combination with a traditional book, taught vocabulary to Group A. Group B students, without the assistance of a teacher, learned vocabulary by using an app on a mobile device. A teacher, in combination with an app on a mobile device, taught Group C. Students in Group B and Group C were given a questionnaire. They were also asked to respond to an interview to help document their experience with the device. The study used a mixed-method design using both qualitative and quantitative methodologies and a pretest-posttest experimental design. The result shows that while most students revealed that they prefer to learn from a conventional printed-based book, the test scores indicate that learning from mobile devices enhances their learning, both when used independently and when combined with a teacher's instruction.