This study investigated how caregivers modulate their speaking rate according to children’s lexical knowledge and the context of the interaction, and how such adjustments affect children’s word learning. We studied a semi- naturalistic corpus where caregivers talked about different toys with their 3-4 years old children. The toys were known or unknown to the child, and present or absent from the environment. We found that caregivers talked about unknown toys with a slower speaking rate than known ones. When toys were absent, caregivers also tended to slow down for the toy’s name, although they produced the whole utterance faster. Crucially, the results of a subsequent recognition task for children showed that caregivers’ greater adjustment in speaking rate between known and unknown words predicted better immediate learning. Our findings suggest that caregivers modify their speaking rate in a helpful manner when the situation is more demanding, which assists children in word learnin