The processes and the cues determining the orthographic structure of polysyllabic words remain far from clear. In the present study, we investigated the role of letter category (consonant vs. vowels) in the perceptual organization of letter strings. In the syllabic counting task, participants were presented with written words matched for the number of spoken syllables and comprising either one vowel cluster less than the number of syllables (hiatus words, e.g., "pharaon") or the same number of vowel clusters (e.g., "parodie"). Relative to control words, readers were slower and less accurate for hiatus words, for which they systematically underestimated the number of syllables (Experiment 1). The effect was stronger when the instructions emphasized response speed (Experiment 2) and when concurrent articulation was used (Experiment 3), and the effect did not stem from phonological structure (Experiment 4). Furthermore, hiatus words were more slowly and less accurately pronounced than control ones (Experiment 5). Finally, in lexical decision, opposite effects occurred as a function of word length, with shorter words producing a facilitatory effect and longer words showing interference (Experiment 6). Taken together, the results show that perceptual units extracted from visual letter strings are influenced by the orthographic status of letters. We discuss the implications of such findings in view of current theories of visual word recognition. (Contains 10 tables and 2 figures.)