Modern Japanese exhibits apparently irregular allomorphic behaviour amongst a subset of bimoraic S[ino]-J[apanese] morphemes, those with a final mora in -/ki/, when appearing as the initial morpheme in a SJ bimorphemic compound whose second morpheme is /k/-initial. Detailed examination of synchronic and diachronic written corpora concludes that what is being witnessed is not irregularity as claimed in previous research, but homomorphemic diffusion, a process akin to lexical diffusion operating on a homomorphemic level. The independent status of homomorphemic diffusion is lent further weight by the phenomenon’s conforming to Bybee’s (2000, 2001, 2002) and Phillips’ (1998, 2001) theories that higher frequency lexemes (here homomorphs) tend to be affected earlier and more thoroughly in the case of reductive sound changes. When all the evidence here presented is examined, homomorphs appear to be behaving in lexical diffusionist terms just as individual lexemes or morphemes might be expected to.