This paper is based on a study that used a quantitative method research design to investigate the impact of English academic vocabulary on the reading comprehension ability of Grade 11 English First Additional Language (EFAL) learners in the Fezile Dabi district of the Free State province, in South Africa. Three tests were administered, namely, a Vocabulary Levels Test (VLT) which measured vocabulary breadth, and a Word Associate Test (WAT) which measured vocabulary depth and a reading comprehension test. The results obtained from the analysis of the data indicate that while both depth and breadth of vocabulary knowledge played an important role in the participating learners' reading comprehension performance, depth of vocabulary knowledge seemed to be a better predictor of comprehension ability than breadth of vocabulary knowledge. The results further reveal that depth and breadth of vocabulary knowledge are positively correlated.