It seems that children are born with sophisticated abilities to perceive speech sounds. Phonological short-term memory plays a key role in the process of language acquisition in the storage of unknown phonological forms while more permanent representations are stored in the long-term memory.In this paper we present the results of a study on the influence of two factors in the short-term memory: the degree of perceptibility (phonetic prominence) and the type of syllable structure (structural prominence) of the stimuli used. The results showed a larger influence of the latter at the expense of the amount of phonetic substance. This reinforces the importance of the role of syllabic structure in the early stages of language acquisition, used as a clue for learning the stress rules on speech and, subsequently, in the process of learning how to read.