This article looks at variation in the distribution of /j/ in post-tonic syllables in Middle Welsh. It extends previous studies by looking at variation at the level of the individual lexical item, using data from a stylistically and lexically relatively homogeneous group of law manuscripts from both north and south Wales. Many items show no variation, appearing either with /j/ or without /j/ in all texts. Variable items show different patterns of distribution: for some items, /j/-full forms are restricted to northern texts, and even there compete with /j/-less forms; for other items, the /j/-full forms dominant in the northern texts are found alongside /j/-less forms even in the south. With frequent items, it seems clear that the overall patterns closely resemble those found with cases of lexical diffusion of linguistic innovations. In addition to documenting the patterns of variation, this article makes some proposals as to how they may have arisen. It is suggested that, in the items investigated closely here (plural suffixes and synchronically monomorphemic items), two processes play the major role: a sound change deleting /j/ in the onset of post-tonic syllables, which diffuses south-to-north; and analogical extension of /j/ into the -eu and -oed plural suffixes, restricted to northern varieties.