In the present research article we present an analysis of the syntactic argument alternation which occurs with the so-called aspectual verbs or aspectualizers. These items occur in two types of sentence, transitive or intransitive, which can be exemplified by pairs of sentences such as a professora comecou a aula ‘the teacher started the class’/ a aula comecou ‘the class started’. First of all, we argue that this alternation is not the causativeinchoative alternation, which occurs with change of state verbs, such as quebrar ‘break’ (o ladrao quebrou o vidro da porta ‘the thief broke the door glass’/o vidro da porta quebrou ‘the door glass broke’). We show how the analyzed phenomenon can be distinguished from the causative-inchoative alternation and we present pieces of evidence to support our claim. In what follows, we propose, following other authors, that aspectual verbs are operators and monoargumental predicators. In our analysis of the alternation of these items, we propose, then, that these verbs can have a derived transitive form, depending on the semantics of their only argument. As aspectualizers are operators over eventualities, their arguments must necessarily denote an eventuality. If such arguments are also predicates, their arguments will be able to appear in syntax in the subject position of the aspectual verbs, deriving a transitive sentence (just as happens with auxiliaries and raising verbs). Thus, we conclude that the argument alternation at stake is not an argument alternation of the aspectual verb per se, but an argument alternation of the embedded predicate, the argument of the aspectual verb.