This dissertation, "Using Mobile Instant Messenger (WhatsApp) to Support Second Language Learning" by Wing-hong, Lai, 賴永康, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Immersion is an acclaimed method for second language acquisition, but is not available to majority of students. The idea of this study is to create Mobile Immersion on smartphone using Instant Messenger, WhatsApp. 45 Form 1 (7th Grade) students participated in a 3-month experiment. They were divided into two big groups: one with Mobile Immersion (Mobile Group) and the other without (Control Group). A vocabulary list of 200 high-frequency verbs was the main learning material. Net Gain in vocabulary score between pre-test and post-test was used to measure their learning progress. The statistics show no significant difference between the two groups' means. However, within Mobile Group, there is significant correlation between individual's chat frequency and vocabulary gain. Furthermore, chat histories of participants reveal a possible theme that has implications for researchers and educators. A questionnaire is also used to survey participants' attitudes towards the Mobile Immersion. This paper shares the experience from the case study and draws insights into a new direction contributing to the knowledge base of Second Language Acquisition and Mobile-Assisted Language Learning. DOI: 10.5353/th_b5396453 Subjects: Second language acquisition - Computer-assisted instruction