Case mixing is a technique that is used to investigate the perceptual processes involved in visual word recognition. Two experiments examined the effect of case mixing on lexical decision latencies. The aim of these experiments was to establish whether different case mixing patterns would interact with the process of appropriate visual segmentation and phonological assembly in word reading. In the first experiment, case mixing had a greater effect on response times to words when it led to visual disruption of the multi-letter graphemes (MLGs) as well as the overall word shape (e.g. pLeAd), compared to when it disrupted overall word shape only (e.g. plEAd). A second experiment replicated this finding with words in which MLGs represent either the vowel (e.g. bOaST vs. bOAst) or the consonant sound (e.g. sNaCK vs. sNAcK). These results confirm that case mixing can have different effect depending on the type of orthographic unit that is broken up by the manipulation. They demonstrate that graphemes are units that play an important role in visual word recognition, and that manipulation of their presentation by case mixing will have a significant effect on response latencies to words in a lexical decision task. As such these findings need to be taken into account by the models of visual word recognition.