According to theoretical accounts, Romance languages differ with respect to the pronunciation of vowel sequences such as /ia/ and /io/. Italian produces these sequences as diphthongs, i.e. /ja/ and /jo/, while Portuguese prefers hiatuses. Spanish and Romanian are claimed to use a mix of diphthongs and hiatuses. These accounts are based on phonological criteria or on small samples of carefully read isolated words. This study proposes to investigate the realisation of /ia/ and /io/ in large corpora of fluent speech, focusing on their acoustic properties along the whole formant trajectory. The results of the functional and statistical analyses show extensive acoustic variation with respect to the duration of the sequences as well as their formant dynamics. The languages clearly differ from one another, but the analysis shows that there remains more to learn about the distinction between diphthongs and hiatuses. We discuss the inclusion of further factors in future investigations.