Different kinds of prepositional phrases are usually included in the grammar of Spanish, obviously as a result of a grammaticalization process. The aim of this paper is to deal with the problems that arise from the categorial, pragmatic and informative status of these elements. I have recently analyzed some of these structures (a propósito de, en cuanto a, en lo que se refiere a, etc.) as discourse markers of topicalization (Cifuentes, 2001b). For this reason I will complete my research on this elements with what is called substractive phrases (excepto, salvo, menos, a excepción de, con la excepción de, aparte de, fuera de, etc.), which obviously will lead us to take into account the additive ones too. These elements have been traditionally considered —at least the most representative ones— imperfect prepositions, and it is said that they are imperfect because they can function as conjunctions as well. And, at this point, the analysis turns more difficult to understand, because their discourse analysis as markers remains undetermined with their categorial analysis —allegedly adverbial—. The synchronic analysis of these structures from the point of view of grammaticalization offers us new paths for their analysis. On the other hand, we will show how all these particles point out the sentence focus, which turns out to be, particularly, a contrastive focus. This explanation can be enriched with another perspective, that of the semantic relationship that the adjunct phrase holds with the considered expressions and the main sentence. We will consider this a quantifying relationship. The categorial status of all these elements is quite difficult to determine, since some of them come from a reanalysis of prepositive elements (aparte, fuera de, hasta), so they allow other similar constructions to appear in the scheme (a excepción de, con la excepción de); and others are a result of a grammaticalization process —mainly from absolute participle constructions— which make possible Sancho Cremades’ idea to consider them hybrids. As a conclusion, I will propose an analysis of the mentioned structures as substractive quantifiers that function as grammatical markers. From the point of view of their informative value, they point out the sentence focus; and their categorial status is a hybrid, as a result of the different, persistent stages of grammaticalization.