The aim of this article is to analyze the syntactico-semantic behaviour of a part of verbal lexicon well delimited in Spanish, that corresponds to the minimal structure verb + se + prepositional syntagma as a stable form in verbs like acordarse de, quejarse de, fijarse en, etc. We interpret the pronominal form as a middle voice morpheme that constantly establishes the limits of process or action within the domain of subject, and clearly maintains the difference from other verbs in which the pronominal form has other functions. The prepositional syntagma must be considered as a semantically obligatory element, syntactically performable in every case as a prepositional form, and pragmatically performable within the relations of enunciation.