The aim of this work is to discuss the occurrence of the preposition por meaning purpose in texts in Portuguese language from XIII to XX centuries, observing the ocasions that this preposition have been gradually replaced by another one: the preposition para. It’s noticeable that, in older texts, occurrences of preposition por meaning purpose are majority, whereas, in most recent texts, preposition para occurs more frequently. The use of preposition por meaning purpose became only a matter of style. We intended to understand the different meanings of the prepositions we’re analyzing, through illustrations from Brazilian authors from the XIX century, Machado de Assis, José de Alencar and Lima Barreto; and from the XX century, Jorge Amado and José Lins do Rego. The particular meaning of purpose in preposition por has not been found in any author from the XX century. We intended to demonstrate the process of continuous disappearance of proposition por meaning purpose throughout the XX century. This work follows a historical approach. It adopts a functionalist perspective, and the collecting of data is based on Labov’s variationist sociolinguistics, as introduced in Brazil by Tarallo (1986 e 1990).