This study describes and explains the paradox of related languages in contact that show signs of both linguistic divergence and convergence. Seventeen distinct languages are spoken in the northernmost islands of Vanuatu. These closely related Oceanic languages have evolved from an earlier dialect network, by progressive diversification. Innovations affecting word forms — mostly sound change and lexical replacement — have usually spread only short distances across the network; their accumulation over time has resulted in linguistic fragmentation, as each spatially-anchored community developed its own distinctive vocabulary. However, while languages follow a strong tendency to diverge in the form of their words, they also exhibit a high degree of isomorphism in their linguistic structures, and in the organization of their grammars and lexicons. This structural homogeneity, typically manifested by the perfect translatability of constructions across languages, reflects the traditions of mutual contact and multilingualism which these small communities have followed throughout their history. While word forms are perceived as emblematic of place and diffuse to smaller social circles, linguistic structures are left free to diffuse across much broader networks. Ultimately, the effects of divergence and convergence are the end result, over time, of these two distinct forms of horizontal diffusion.