This study focuses on the acquisition of verbal aspect in Portuguese, in the context of theory of language acquisition that aims at characterizing the way children perceive grammatically relevant information at the interface levels between language and articulatory/phonetic and conceptual/intentional systems - phonetic and semantic interfaces, respectively. It is based on the assumptions of the Minimalist Program (Chomsky, 1995; 1999) and on a theoretical approach to language acquisition requiring articulation between a linguistic model of language and psycholinguistic models of language processing (Corrêa, 2005a; 2006). The working hypothesis is that the child initially recognizes aspect as a formal feature on the basis of morphophonological variations in the closed class of verbal affixes (marked vs. unmarked forms), when tense and aspect are undissociated. Distinctions concerning perfectivity would be established later on, on the basis of processing at the semantic interface. The extent to which information pertaining to semantic features of the verbal root (such as telicity) interacts with information concerning perfectivity provided by variation in the form of verbal affixes is considered. An experimental methodology is developed, which is intended to enable the perception of information concerning tense/aspect from the phonetic and the semantic interfaces to be dissociated. The novelty detection paradigm, traditionally used in the study of conceptual development, was explored here so that novelty could be detected both in the perception of the form of affixes and in the perception of a visually presented event. A pilot- experiment conducted with 18- 28 month old children is reported. An acting-out experiment, conducted with 3-5 year olds, is also presented, in which telecity and perfectivity were the independent variables. Questions concerning methodology for the study of processing at the semantic interface are considered, and the results are discussed in relation to children´s spontaneous production data regarding aspectual contrasts and current hypotheses on the role of semantic information in the establishment of grammatical distinctions.