This study examined potential differences in vocabulary found in picture books and adult's speech to children and to other adults. Using a small sample of various sources of speech and print, Hayes observed that print had a more extensive vocabulary than speech. The current analyses of two different spoken language databases and an assembled picture book corpus replicated and extended these findings. The vocabulary in picture books was more extensive than that found in child-directed speech (CDS) and even adult-directed speech (ADS). The likelihood of observing a rare word not contained in the most common 5,000 words in English was more likely in a corpus of picture books than in two different corpora of CDS. The likelihood of a rare word in the picture books was even greater than that found in ADS. It is proposed that these differences are more indicative of informal versus formal language rather than the spoken versus written modalities per se. Nonetheless, these results highlight the value of rich read-aloud experiences for vocabulary development and potentially for reading comprehension once written language is acquired. These findings are described in terms of a distinction between formal and informal language, which has implications for views of literacy, cognitive and linguistic development, and learning to read.