Many people with aphasia show effects of word length on the accuracy of their word production. However, it is unclear from previous research whether this is an effect of number of phonemes, number of syllables or the complexity of the syllable structure (number of consonant clusters), as these factors are usually confounded. This paper is the first to attempt to dissociate the effects of these three factors on word production in a case series of nine English-speaking aphasic individuals. Using carefully controlled stimuli and analysis using logistic regression, the effects of these highly intercorrelated variables can be distinguished. Significant effects of number of phonemes were found on word production accuracy in English but there was no evidence for independent effects of number of syllables, number of clusters (syllabic complexity), or syllable frequency (once the intercorrelations between these variables had been controlled). It is argued that these data are consistent with any theory of spoken word production that incorporates a level of processing where phonemes are represented and that level is a potential source of error.