Are we more likely to have tip-of-the-tongue (TOT) states on some words rather than others? We report two experiments in which we manipulate the frequency and phonological distinctiveness of the target words. We show that TOTs are more likely to arise on low-frequency words that have few close phonological neighbours. We further discuss the effect of word length upon lexical access. The data are interpreted in terms of a two-stage model of lexical access in speech production. We argue that TOTs arise because of a transient difficulty in accessing unusual phonological word forms after the corresponding abstract lexical representation has been successfully accessed. Our data fit a partial activation model of the origin of TOTs rather than an interference model. Indeed, we argue that phonological neighbours usually play a supporting rather than a blocking role in lexical access. We conclude that the role of lexical neighbourhoods may have been underestimated in previous research on lexicalization in language production.