The aim of this study is to determine the acoustic properties of hiatuses (vowel-vowel sequences) and diphthongs (glide-vowel sequences) in Spanish and to observe how these properties are modified depending on communicative factors. To do this, two groups of data were used: speech samples gathered from conversations between two speakers participating in the execution of a map task, in which the corpus items corresponded to toponyms, and the reading of the same sequences at a normal speaking rate. The comparison was done phonetically and phonologically: first, diphthongs and hiatuses were analyzed acoustically, studying their duration and spectral dynamics, and later, an inventory of diphthongizations and monophthongizations was made. Results show that hiatuses and diphthongs differ in the temporal and frequential domain: hiatuses have a longer duration and a greater degree of curvature in the F2 trajectory than diphthongs. Differences between the two categories (hiatus and diphthong) exist in both communicative situations, although changes within each category due to the speech situation were also observed: sequences from the map task are shorter and the degree of curvature of their formant trajectories is lower than for the reading task. We have also found that vowel-vowel and glide-vowel sequences behave differently in the way they are phonetically reduced: a reduction axis can be drawn in which hiatuses become diphthongs, and diphthongs vowels. It is concluded that hiatus and diphthong are two phonetic categories which can be described on the basis of their acoustic characteristics and are subject, like any other phonetic category, to modifications due to a change in the communicative situation.