This work investigates the relation between different abverbial constructions in Brazilian Portuguese, in order to establish the extent to which syntactic and morphological characteristics of adverbialized adjectives are connected to the potential interchangeability between these constructions and X-mente adverbs in the same syntactic context. We examine first the controversies regarding the conceptualization and classification of adverbs. Then we discuss the relation between class and function, after which we approach the processes of adverb formation in Brazilian Portuguese and the way X-mente forms and adverbial adjectives are described in the traditional literature on the subject. Some functional and generative oriented studies are then presented, in which significant discrepancies are found in the way adverbial adjectives are analyzed. In this study, we consider adverbial adjectives to result from a morphological conversion process because, even though adjectives and adverbs are different lexical categories, they are related to each other. The adverbial adjectives used in the analysis were extracted from a corpus specially formed for this study. The results show that the morphological characteristics of the base word and the degree of informality in the speech act are connected to the potential for having both constructions in Portuguese. Our results also lead to the conclusion that adverbial adjectives normally have an X-mente counterpart with no semantic change.