This paper first describes the prefixation and circumfixation processes of Mirandese word formation and determines the general conditions of phonotactic correspondence between Portuguese and Mirandese. It then analyses the permeability of Mirandese to Portuguese in word formation, specifically concerning these affixation processes, and goes on to identify the specific phonological conditions that concern the allomorphy of each affix while quantifying their morphotactic transparency. Using Natural Morphology as a framework, data from this analysis demonstrate a relationship between morphotactic transparency and the actualisation of either specific allomorphic conditions or the general conditions of phonotactic correspondence.