The integration of letter and speech sounds is essential when learning to read. Recent studies suggested that the integration of letters and speech sounds takes place automatically. In our study, we wished to determine the neural networks associated with an automatic integration of written and heard syllables in literate adults. We used the mismatch negativity (MMN), an index of automatic change detection in the brain. Subjects were presented with several speech sounds in an experimental and a control condition. In addition, in the experimental condition syllables were presented on the screen synchronously with the sounds, whereas in the control condition scrambled images of the written syllables were used. During stimulation, the subject’s primary task was to press a button to targets presented on the screen and ignore interspersed distractors. Changes in auditory stimuli elicited an MMN, in both conditions. The MMNs were diminished in the experimental condition compared with the control condition. The decrement of the MMN was most prominent for phonologically relevant features in the subject’s native language. The implications of the results are discussed.