The present study presents an experiment in which online acronyms, formed from common fixed phrases or formulaic expressions, and in common usage in English medium computer-based communication, were presented to Korean university-level learners placed into either a control group or treatment group which was given instruction into the expansions and meanings of the acronyms. Their knowledge of the target vocabulary was measured using the Vocabulary Knowledge Scale (VKS) test in a pre-, post-, delayed post-test format. The pre-test results showed relatively little awareness or familiarity with the target online acronyms for either group. Repeated measures ANOVA analysis did not show differences between pre-test and immediate post-test scores for the control group, although the delayed post-test did show a significant improvement. The treatment group showed significant and robust gains in both immediate and delayed post-tests. Comparison between the groups was done by one-way ANOVA. This showed significant differences in gains between control and treatment groups, with a large effect, suggesting that brief explicit instruction sessions could result in large gains. Implications of the study findings for educators and learners are discussed.