The main goal of this research is to empirically study Portuguese and Spanish infinitival constructions with specified subjects in adverbial contexts, on the basis of statistical analyses of corpus and experimental data. Within a functional-cognitive paradigm, the first objective is to offer a state of the art on three theoretical issues that are relevant for these constructions: (1) the hybrid nature of the infinitive, (2) the subject as a multifaceted prototype concept and (3) adverbial clauses as a heterogeneous group of constructions showing more or less autonomy with respect to the main clause. The theoretical investigation is followed by a twofold extensive empirical study. The first study focuses on the use of the Portuguese inflected infinitive in adverbial contexts in which the uninflected infinitive is equally well-formed (i.e. in cases of co-reference between the main subject and the infinitival subject), as in Vamos à escola para aprender (uninflected infinitive) / aprendermos (inflected infinitive) coisas novas. Results from corpus data are compared with data from a self-paced reading experiment. It turns out that the use of the inflected infinitive in these contexts can be accounted for by reduced mental accessibility of the infinitival subject, but also by the syntactic-semantic autonomy of the clause and by the explicitly verbal nature of the infinitive. The second case study is a comparative corpus investigation on Spanish and Portuguese infinitives with proper subjects (al entrar Jjuan (Sp); pare ele cá estar (Pt)). The Portuguese constructions turn out to be more clause-like, having a more prototypical subject and an infinitive that is closer to the verbal prototype, while Spanish infinitives with proper subjects are more nominal in nature.