We present an MEG study of homonym recognition in reading, identifying effects of a semantic measure of homonym ambiguity. This measure sheds light on two competing theories of lexical access: the "early access" theory, which entails that lexical access occurs at early (pre 200 ms) stages of processing; and the "late access" theory, which interprets this early activity as orthographic word form identification rather than genuine lexical access. A correlational analysis method was employed to examine effects of the lexical frequencies of distinct words that share the same orthography (homographs) on brain activity. We find that lexical frequency did not affect processing until after 300 ms, while earlier activation was primarily modulated by orthographic form frequency. (Contains 4 figures and 1 table.)