During the second year of life, infants develop a preference to attach novel labels to novel objects. This behavior is commonly known as "mutual exclusivity" (Markman, 1989). In an intermodal preferential looking experiment with 19.5- and 22.5-month-olds, stimulus repetition was critical for observing mutual exclusivity. On the first occasion that a novel label was presented with 1 familiar object and 1 novel object, looking behavior was unsystematic. However, on reexposure to the same stimuli, 22.5-month-olds looked preferentially at the novel object "prior" to the re-presentation of the novel label. These findings suggest a powerful memory mechanism for novel labels and objects, enabling mutual exclusivity to emerge across repeated exposures to potential referents. (Contains 6 figures and 5 footnotes.)