This study investigated individual differences in vocabulary acquisition in 8-9-year-old children with impaired reading comprehension. Despite fluent and accurate reading, age-appropriate phonological skills and normal nonverbal ability, poor comprehenders are poor at understanding what they have read, and they show a variety of oral language weaknesses in the non-phonological domain. In this experiment, children were taught to associate new phonological forms to pictures of novel objects (phonological learning) and taught semantic information about the objects (semantic learning). Poor comprehenders needed as many trials as control children to learn phonological forms, suggesting they are well-equipped with the skills needed to learn labels for new objects. However, their knowledge of the meaning of the new words was relatively weak and not well-consolidated over time. These findings suggest that the source of poor comprehenders' difficulties with lexical learning may be localized to the semantic rather than phonological component of vocabulary learning. Implications for understanding the nature of the relationship between reading comprehension and vocabulary skills are discussed.