This article addresses the status of homogeneous diphthongs in Spanish (those formed of two high vocoids, usually spelled iu or ui) to try to determine which vocoid acts as the syllable nucleus and which is the glide. It has generally been assumed that the order is glide-vowel (rising-like diphthong) but with little factual substantiation. A study of examples of rhyme from different ages in Spanish poetry and theatre in verse confirms the strong tendency for these diphthongs to act as rising-like.