The number of learners seeking to acquire an L2 by means of apps is ever-increasing. This study examined the effectiveness of using the app "Mondly" for the acquisition of L2 English vocabulary while accounting for differences in the acquisition of receptive and productive aspects of word knowledge. Twelve Danish young learners (aged 10-11) used "Mondly", installed on their personal devices, as their sole English curriculum for 4 weeks. The results showed a significant increase in vocabulary knowledge. These gains were particularly notable in regards to productive knowledge of the form-meaning connection and areas of word knowledge relating to use. While these learning gains correlated positively with the frequency of appearance of each target word in "Mondly's" lexical inventory, they correlated negatively with the participants' use of the app. We hypothesise that this was because additional use of the app with inadequate repetitions of target words leads to the creation of weak synaptic pathways which interfere with recollection of receptive knowledge of the form-meaning connection. The study indicates that "Mondly" may be effective as a tool for acquiring L2 vocabulary. However, explicit attention should be given to the number of repetitions of each target word in order to ensure adequate entrenchment.