This study investigated several factors presumed to influence the intelligibility of song lyrics. Twenty-seven participants listened to recordings of musical passages sung in English; each passage consisted of a brief musical phrase sung by a solo voice. Six vocalists produced the corpus of sung phrases. Eight hypotheses derived from common phonological and prosodic principles were tested. Intelligibility of lyrics was degraded: (i) when archaic language was used; (ii) when words were set in melismatic rather than syllabic contexts; (iii) when the musical rhythm did not match the prosodic speech rhythm; and (iv) when successive target words rhymed. Intelligibility of lyrics was facilitated: (i) when words contained diphthongs rather than monophthongs; (ii) when a word from an immediately previous passage reappeared; (iii) when a syllabic setting of a word was preceded by a melismatic setting of the same word. No difference in word intelligibility was observed between musical-theatre singers and opera singers. THE most popular forms of music typically involve the human voice. In nearly all cultures, singing is one of the preeminent forms of music making. Although singing often employs nonsense words or vocables, most singing employs lyrics conveying a narrative or poetic text. Nevertheless, concertgoers and music listeners frequently note that it is difficult to recognize or comprehend musical lyrics. In earlier work, we measured the intelligibility of sung English text. For advanced voice student performances, sung passages showed a 75 percent decrease in intelligibility compared with spoken counterparts (Collister & Huron, 2008). The number of errors in lexical item identification for sung words proved to be 7.3 times the number of errors for spoken words. Post hoc analyses of the word-identification errors identified a number of common confusions. Three-quarters of phonetic errors involved the misidentification of consonants. Among vowels, roughly one-third of identification errors arose from centralization, as when " beat " is misheard as " bet. " Another phonetic error was diphthongization in which monophthongal vowels were misheard as diphthongs, such as mishearing " toe " as " toy. " In the current study, we attempted to further isolate some possible factors that may either confound or facilitate vocal intelligibility in sung passages. Specifically, we put forward the following hypotheses:  H1: High frequency (vernacular) words are more intelligible than low frequency (archaic) words.  H2: Words containing diphthongs are less intelligible than words without diphthongs.