The purpose is to detect whether first year university students of English enrolled in an English Study Program benefit from a contextualized vocabulary instruction with only three and six encounters. The research was carried out using the least known terms from the General Service List. As these terms were high in frequency, it was hypothesized that fewer encounters than reported in the literature in the field (Rott, 1999) would suffice. We found suitable authentic contexts in the British National Corpus. In order to find out which type of instruction was more effective (three vs. six encounters), an empirical study with a Pre-test/Post-test design was carried out. We found that with six encounters the benefit was significantly better than with 3 (mean=6.19 vs. 3.13). Nonetheless, a significant difference was found with both types of encounters. This finding has relevant pedagogical implications related to the number of encounters needed by students in order to acquire a high frequency vocabulary. It is advisable to provide a large number of encounters when teaching this type of vocabulary; if it is not possible to expose students to numerous encounters three could be enough, as with three there are substantial gains. The number of encounters is evidently a strategy to be considered when this vocabulary is taught.