The goals of the present paper are: to improve the reconstruction of Proto-Mojeño (PM) by reconstructing a synchronic morphophonological alternation; to provide a clearer picture of the relations between PM and some of its closest relatives and to better understand the historical development of the Trinitario dialect, arguably the least conservative of the attested varieties of Mojeño. I show here that an antecedent stage of PM, Pre-PM, was subject to a contextual merger (primary split or split-merger) of *k and *s, yielding a morphophonological alternation *k ~ *s in PM. Apparent exceptions to the Pre-PM change *k>s provide evidence for the reconstruction of a non-palatalizing contextual vowel, possibly *ɨ, that was merged with *i in PM and with e in Terena, possibly PM’s closest relative. A preliminary comparison of certain verb stem formatives and deverbal nominalizers in PM and Terena is also presented. Finally, I show that a context-dependent development involving k in the Trinitario dialect, this time a secondary split yielding the fricative ç, resulted in alternations k ~ ç, thus creating a second, diachronically separable layer of palatalization/coronalization in the phonology of Trinitario.