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Most intelligibility tests are based on the use of monosyllabic test stimuli. This constraint eliminates the ability to measure the effects of lexical stress patterns, complex phonotactic organizations, and morphological complexity on intelligibility. Since these aspects of lexical structure affect speech production (e.g., by changing syllable duration), it is likely that they affect the structure of acoustic-phonetic patterns. Thus, to the extent that text-to-speech systems fail to modify acoustic-phonetic patterns appropriately in polysyllabic words, intelligibility may suffer. This means that while most standard intelligibility tests may accurately estimate the intelligibility of monosyllabic words, this estimate may not generalize as well to predict the intelligibility of words with more complex lexical structures. The present study was carried out to measure how words varying in lexical complexity differ in intelligibility. Monosyllabic, bisyllabic, and polysyllabic words were used varying in morphological complexity (monomorphemic or polymorphemic). Listeners transcribed these stimuli spoken by two human talkers and two text-to-speech systems varying in speech quality. The results indicate that lexical complexity does affect the measured intelligibility of synthetic speech and should be manipulated in order to accurately predict the performance of text-to-speech systems with unrestricted natural text.