Abstract This article proposes a sonority-based approach to Sino-Japanese morphophonemic alternations. An earlier OT analysis by Kurisu (2000) sees Sino-Japanese obstruent gemination as the result of minimization of the number of syllables induced by a gradient alignment constraint. However, gradient evaluation of constraints runs counter to McCarthy’s (2003) argument that OT constraints are categorically evaluated. The goal of this article is to resolve this problem by developing a unified OT analysis of obstruent gemination and syllabification of stem-final nasals as codas with reference to onset sonority distinctions. Less sonorous segments have a stronger preference to syllabify as onsets than do more sonorous segments. This preference relationship between segment classes and syllabification has been formalized as the Onset Sonority Hierarchy (Prince & Smolensky 1993; Baertsch 2002; Smith 2003; Moreton et al 2006; etc). By taking the position that local conjunction of onset sonority and peak constraints plays the central role in creating the observed syllabification pattern through interaction with faithfulness and other markedness constraints, this analysis also makes it possible to view the segmental restrictions on the onsets of the second syllables of Sino-Japanese stems as a concomitant onset sonority effect.