Although awareness of the underlying phonological structure of a language is said to be a prerequisite in learning to read, there is still much debate as to which phonological units are the best predictors. The syllable plays an important role in accounting for various phonological phenomena observed cross-linguistically, and studies have shown its importance in promoting reading abilities. However, in the case of Tokyo Japanese, the role of the syllable is inconspicuous compared to the mora unit. This is due to the fact that in practice, the syllable often overlaps with the mora, exceptions arising from the syllables that contain the so-called 'special moras', i.e. the heavy syllables containing two moras. Traditionally, the main focus of attention in Japanese had been laid on different behaviours observed between the plain 'normal' moras and the 'special' moraic phonemes. However, in recent years, studies conducted have been focused on the different phonological patterning observed within the moraic phonemes themselves. In order to better understand the characteristics of these moraic phonemes, we report on our findings based on reversal tasks administered to elementary school students. The result of the experiment suggests that the degree of independency of these moraic phonemes in terms of phonological make-up seems to have an effect on the error patterns observed.