Adults often learn new semantic information in face-to-face communication with other adults (e.g., teachers, colleagues). More knowledgeable individuals provide an ensemble of multimodal behaviours that can shape the information that their interlocutors learn. Using the naturalistic ECOLANG corpus of dyadic conversations, we ask whether multimodal behaviours (pitch, speaking rate, representational gestures, points, object manipulations, and gaze) support adults' semantic learning of unknown objects above and beyond verbal properties of utterances (number of utterances, lexical diversity, mean length of utterances, concreteness) and learners' individual differences (vocabulary, working memory). We found that individual differences, pointing and object manipulations affected learning, with verbal and multimodal factors also interacting to predict adult semantic learning. Our results highlight the relevance of accounts of multimodal learning in adulthood and the importance of considering naturalistic interaction in its complexity to understand the factors that influence adult learning.