The aim of the present essay is to analyse several types of Spanish prefixes that, having like all the others prefixes a basically semantic value, imply at the same time the modification of the sentence transitivity. This transitivity is named "weak tansitivity", since it entails obvious limitations in relation to the most common diathetic variations: causativity, reflexivity and others. The diathesis implicated in this field are usually progressive (causative). The actantial variation is done frequently with the step of one to two actants (el barco flota / los marineros han reflotado el barco) and unfrequently with the step of two to one actant (embocó el hoyo diez / desemboca en el Atlántico). The starting point is the natural-topo...