This dissertation presents an experimental study of the delimitation of adjectives as a lexical category in the acquisition of Brazilian Portuguese. It reconciles a psycholinguistic approach to language acquisition with a minimalist conception of language (Chomsky, 1995-2001). According to the phonological bootstrapping hypothesis (Morgan & Demuth, 1996; Christophe et al., 1997), it is assumed that infants are sensitive to closed class elements (as determiners and affixes) in the processing of the phonetic interface. The functional category D is considered to be fundamental to the distinction between nouns and adjectives, and derivational suffixes are viewed as contributing to the representation of the adjective as a categorical feature in the lexicon. According to the syntactic bootstrapping hypothesis (Gleitman, 1990), it is assumed that the parsing of adjectives in DP and small clause contexts, together with the assumption that DPs refer to objects/entities, allow the representation of the adjectives as words that present a property or an attribute of a given referent. And the role of the canonical word order in the distinction between nouns and adjectives, in the absence of specific morphological information, is evaluated. In this study, the linguistic literature on adjectives is extensively reviewed. This review contributes to the identification of the demands that the delimitation of adjectives present to children, taking into account the specific properties of Portuguese. Four experiments were conducted: three with children, making use of the object selection with pseudo-words paradigm, and one with adult speakers of BP. In the latter, a task was envisaged in which pseudo-adjectives were presented in order for the property of a hidden object/character to be guessed. The speaker’s intuitive knowledge of the meaning of derivational suffixes was expected to guide the accomplishment of this task. The two first experiments were conducted with 12- 22 month old children, and the last one, with 2-3 and 4-5 year olds. In Experiment 1, the role of determiners and derivational affixes (forming adjectives) in the parsing of adjectives in predicative structures was evaluated, independently of the semantic properties of those affixes. In Experiment 2, the role of the canonical word order within the DP and derivational affixes in the bootstrapping of the representation of adjectives in Portuguese was investigated. Experiment 3 explores the native speaker knowledge of the semantic features of the adjective forming derivational affixes -oso e -ento. In Experiment 4, children’s representation of these very features is assessed. The results of the experiments reported here are compatible with the working hypothesis, namely, that children make use of syntactic and morphological information in the delimitation of adjectives as a lexical category. They also reveal that the semantic properties of derivation affixes forming adjectives are already available by the age of two. This work contributes to a theory of language acquisition that explores early processing at the language interfaces with perceptual and conceptual systems, and in which the role of closed class elements (including derivational affixes) and functional categories is emphasized. It also introduces an experimental methodology that enables to access the adult speaker’s intuitive knowledge of derivational morphology.