The Arabic orthographic system is an example of an abjad; consonants and long vowels are written, while short vowels are generally omitted except in specific contexts. Despite the fact that this situation is relatively well-known, the specific role that diacritics play in the processing of written Arabic words is as yet relatively understudied. The present study explores the effects of diacritics on 41 native speakers' performance on a lexical decision task. In addition to lexicality and the presence or absence of diacritics, the experimental design also incorporated an additional factor of ambiguity. This term refers to the fact that some written forms without diacritics may represent multiple, non-homonymic words. Analysis of the resulting data yields three main findings. First, consistent with earlier studies, response times to words were faster than those to pseudo-words. Second, and most significantly, despite the fact that word-forms written with diacritics provide more information than those written without, the presence of diacritics greatly slowed the response times of this group of native speakers. Finally, effects related to ambiguity proved to be relatively weak.