General academic words are those which are typically learned through exposure to school texts and occur across disciplines. We examined academic vocabulary assessment data from a group of English-speaking middle school students (N = 1,747). We tested how word frequency, complexity, proximity, polysemy, and diversity related to students' knowledge of target words across ability levels. Our results affirm the strong relation between vocabulary and reading at the individual level. Strong readers were more likely to know the meanings of words than struggling readers were, regardless of the features of the academic words tested. Words with more meanings were easier for all students, on average. The relation between word frequency and item difficulty was stronger among better readers, whereas the relation between word complexity and item difficulty was stronger among less proficient readers. Our examination of academic words' characteristics and how these characteristics relate to word difficulty across reading performance has implications for instruction.