This work investigates lexical processes underlying the formation of não- X nouns and adjectives in Portuguese, taking into account both morphological and semantic aspects. We approach first the two main word formation possibilities, derivation and compounding, as they correspond to the possible alternative hypotheses for the use of não in word formation, since this lexical item is traditionally considered only as an adverb. Traditional approaches to the topic are described and considered insufficient to explain the phenomena in a satisfactory manner. More recent descriptive proposals on the subject are analyzed and their controversies commented upon. The data favor the analysis of não-X constructions in Portuguese as a case of derivation, specially in what concerns the need to establish criteria to classify não-X constructions as new words. The data were collected from two corpora – the CORPOBRAS PUC-RIO and the CORPUS NILC, which were compared as for the phenomena in question. The work is based, to a great extent, on the mechanisms and methodologies proposed by Corpus Linguistics researchers, with the use of computational tools and the conception of corpora as defined by CL authors. As a result of the analysis, it is proposed that not every não-X occurrence constitutes a new lexical item, since many of the studied examples reveal predominance of syntactic aspects. As for the semantic question, it has been observed that the meanings of the não-X constructions are partially predictable, in spite of some exceptions. The research also indicates the presence of some stylistic possibilities for the use of the não X construction.