Individuals with aphasia have more problems detecting small differences between speech sounds than larger ones. This paper reports how phonemic processing is impaired and how this is influenced by speechreading. A non-word discrimination task was carried out with "audiovisual", "auditory only" and "visual only" stimulus display. Subjects had to decide whether two presented stimuli were the same. Six aphasic subjects with speech sound processing difficulties and 14 non-brain-damaged control subjects participated in this study. It was found that the aphasic subjects have difficulties in discriminating pairs of non-words, which are more profound for small differences. Differences in "voicing" were least often detected and therefore seem most difficult to perceive. This implies different processing of the phonetic dimensions in speech sound perception. Performance improved when speechreading was possible. As this improvement is not based on differences in place of articulation only, theories of audiovisual processing need to be revised. (Contains 7 tables, 4 figures and 1 note.)