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.