Several empirical lines of investigation support the idea that syllable-sized units may be involved in visual word recognition processes. In this perspective, the present study aimed at investigating further the nature of the process that causes syllabic effects in reading. To do so, the syllable frequency effect was investigated in French using event related potentials while participants performed a lexical decision task (Experiment 1). Consistent with previous studies, manipulating the frequency of the first syllable in words and pseudowords yielded two temporally distinct effects. Compared to items with a first syllable of low frequency, items with a syllable of high frequency elicited a weaker P200 component, reflecting early sub-lexical facilitation, and a larger N400 component, supposed to ensue from competition between syllabic neighbours. To examine which factors determine the strength of interference during lexical access, regression analyses were conducted on the late temporal window potentials. The inhibitory syllable frequency effect was best predicted by leader strength, that is, the frequency ratio between the most frequent syllabic neighbour and the others. When this variable was directly manipulated while controlling for syllable frequency and number of higher frequency syllabic neighbours (Experiment 2), electrophysiological data confirmed the impact of leader strength. The results are discussed in the context of interactive activation-based models augmented with syllabic representations. (Contains 4 tables and 7 figures.)