The purpose of this study was to examine the factor structure of vocabulary. We believe that not only is vocabulary multidimensional, but depth of vocabulary knowledge should also be assessed with multiple measures since it too, is composed of multiple aspects. Furthermore, to explore the predictive validity of the different aspects of vocabulary knowledge, we assessed the relationship between vocabulary breadth, vocabulary depth, and reading comprehension in adults with low literacy skills. The participants were 103 adults. They completed 12 tasks that have been used in past studies to measure vocabulary breadth, depth, and reading comprehension. We had several important findings. First, we confirmed that all of the assessments were highly reliable for adults with low literacy skills. Second, the results of the factor analysis indicated two distinct vocabulary factors. Finally, both breadth and depth contribute independently to explaining variance in reading comprehension. Implications for vocabulary measurement are suggested.