When multiple adjectives are used to modify a noun, they tend to be sequenced in the following way according to semantic class: value > size > dimension > various physical properties > color. To investigate the neural substrates of these semantic constraints on adjective order, we administered a battery of three tests to 34 brain-damaged patients and 19 healthy participants. Six patients manifested the following performance profile. First, they failed a test that required them to discriminate between semantically determined correct and incorrect sequences of adjectives-e.g., thick blue towel vs. *blue thick towel. Second, they passed a test that assessed their knowledge of two purely syntactic aspects of adjective order-specifically, that adjectives can precede nouns, and that adjectives can precede other adjectives. Finally, they also passed a test that assessed their knowledge of the categorical (i.e., class-level) features of adjective meanings that interact with the semantic constraints underlying adjective order-e.g., that thick is a dimensional adjective and that blue is a color adjective. Taken together, these behavioral findings suggest that the six patients have selectively impaired knowledge of the abstract principles that determine how different semantic classes of adjectives are typically mapped onto different syntactic positions in NPs. To identify the neuroanatomical lesion patterns that tend to correlate with defective processing of adjective order, we combined lesion data from the six patients just described with lesion data from six other patients who we reported in a previous study as having similar impairments [Kemmerer, D. (2000). Selective impairment of knowledge underlying adjective order: Evidence for the autonomy of grammatical semantics. Journal of Neurolinguistics, 13, 57-82.] We found that the most common areas of damage included the left posterior inferior frontal gyrus and the left inferior parietal lobule. Overall, these results shed new light on the neural substrates of the syntax-semantics interface.