In this paper, the capacity of four-month-old infants from monolingual environments to distinguish between two syllable-timed languages is analysed. Catalan and Spanish are both Romance languages which present differences at the segmental level and at the syllable structure level, but show important similarities concerning prosodic structure at the phonological phrase level. Nevertheless, the presence of vowel reduction only in Catalan may determine rhythmic differences which could be detected by infants and used to tell these two languages apart. Two experiments have been run, with normal and low-pass filtered utterances, using a visual orientation procedure with a reaction time measure. Results indicate that infants are able to discriminate even when segmental information has been removed. The distinction seems to be the result of basic differential rhythmic properties between these two languages.