The main objective of this study is to compare the structure of the factitive construction expressing negative causation withdejar/deixar(“to let”) in Ibero-Romance languages. It is generally accepted that Portuguese and Spanish exhibit a high degree of syntactic equivalence. However, the nature of the infinitive in the two languages is quite different, displaying more verbal characteristics in Portuguese than in Spanish. By means of a detailed empirical study, this article examines whether this structural difference has an effect on the syntax of the causatives withdeixaranddejar. Indeed, statistically the selection of the different complement types (finite clause vs. infinitive, with an anteposed or postposed causee) differs substantially in the two languages. A multifactorial analysis shows to what extent the degree of dynamicity of the main constituents, namely the causer, the causee and the caused event, determine the syntactic variation in the two languages and how this variation can be linked to the different grammatical status of the infinitive.