Idiomatic or complex prepositions, phrases that perform the same functions as simple prepositions, are examined in Italian. The analysis is of only one subgroup of idiomatic prepositional phrases, preposition + noun, and looks for classification criteria of theoretical rather than descriptive value. It is concluded that the idiosyncratic behavior of these constructions with respect to the binding theory suggests a categorical modification corresponding to the suppression of the functional and lexical properties of the nominal head. While this conclusion is neither explanatory nor exhaustive, it appears to fix some important points for the empirical and formal identification of these idiomatic constructions and can be seen as a methodological step toward a theoretical definition of idiomaticity itself. (MSE)