Few dictionary studies have examined the effectiveness of freely available online dictionaries (FAOD) offered by search engine companies. This experimental study examines the effectiveness of one FAOD, Naver.com, as an aid to vocabulary acquisition for a group of 87 intermediate-level Korean university students aged 19 to 28 (mean 21.9). The VKS and two multiple choice tests administered after a written recall task, and again one month later, measured vocabulary acquisition. MANOVA using treatment type (Online, Gloss, or Control) as the independent variable and test results as the dependent variables compared vocabulary acquisition. A second line of inquiry examined effects of topical familiarity on vocabulary acquisition. Findings suggest that FAODs are equally effective as a written gloss, and that both significantly outperform using context clues. Evidence is presented that unfamiliar topics may lead to slightly better vocabulary gains over reading familiar topics. Ways these findings can assist learners and teachers are discussed.