In this study an attempt is made to show that, upon close examination, certain phonemic alternations in Mandarin Chinese, which hitherto have been regarded as due to dialectal mixing, can be established as being internal evidence for some old phonological distinctions. The study examines various cases of dissimilation among pre-vocalic (or post-vocalic) segments which post-vocalic (or pre-vocalic) segments have conditioned within syllables during the history of the Chinese language. The discussion is focused specifically on the phonological factors conditioning the alternation of velar vs. palatal initial consonants, and of velar vs. dental final consonants. These alternations are interpreted as a special type of dissimilation—that of medials conditioned by final palatal nasal and stop, which can be reconstructed on the basis of quite independent external evidence. This leads to the reconstruction of a four-way distinction among Ancient Chinese ending consonants: labial, dental, palatal, and velar.