In this paper, it is hypothesized that units larger than the traditional word, such as two-word collocations and phrases, may be stored in the mental lexicon and accessed holistically. Following previously published work on the Network or Usage-based model of lexical storage (Bybee, 1985, 1995), we suggest that the mechanism determining this constituency is the frequency with which items occur together in natural, connected speech: the collocational frequency. The present study uses a word-monitoring paradigm to investigate reaction times to the English function word of in collocations of varying levels of frequency. A significant effect of collocational frequency was observed; response latencies were longer when the target word of occurred in the very frequent collocations, indicating holistic processing of the frequent phrases. Furthermore, response latencies presented here are considerably longer than those reported in previous studies. This is explained as a result of the hypothesized holistic representation as well as a function of the use of stimuli extracted from natural conversation.