Aural vocabulary knowledge (AVK) of high-frequency words is critical for second language listening comprehension. However, learners of English as a foreign language (EFL), despite considerable periods of language study, often do not have the AVK needed for basic communicative competence. A mixed methods approach is applied to determine whether mobile-assisted language learning (MALL) can help solve this problem. First, a cohort of 33 Japanese EFL students with suboptimal AVK were divided into a treatment group (n = 17) and a control group (n = 16). Over a two-month period, the treatment group used a MALL application that enabled self-assessment and teacher monitoring of learners’ AVK of 1,000 frequently occurring English words. A comparison of pre- and post-test scores between the treatment and control group suggested that the MALL intervention was associated with improvements in AVK. Further, data stored by the application enabled an analysis of the words learners found most difficult, and interviews with a sub-group (n = 5) were undertaken. Results suggested that reduced forms and the influence of Japanese phonotactics were key factors limiting the ability to recognize spoken words. Links to resources that can help learners improve AVK of high-frequency words are provided and pedagogical implications are discussed.