Allomorphic alternations are often accounted for by assuming them to be listed as idiosyncratic lexical entries. This paper analyzes the forms of the determiners (definite, indefinite, and demostratives) and definite prepositions across all Italo-Romance varieties, to show that such a view (for a review on accounts for the definite determiner, see Garrapa 2011) may lead to lose crucial generalizations at the morpho-phonological interface. Here, I propose a novel alternative account couched in the framework of Distributed Morphology (Halle & Marantz1993), which makes use of morpho–phonological operations, such as positive instructions (rules), or repairs triggered in response to violations of active filters (Calabrese, 2009). I show that such an account is able to catch the underlying morpho–phonological generalizations and therefore more coherently explain allomorphic phenomena occurring in the D–domain.