This paper reports on the diphthongization of mid/low adjacent vowels in two varieties of Colombian Spanish (Andean and Caribbean). An acoustic analysis on the pronunciation of adjacent vowels [eo], which according to the rule are pronounced as hiatus, was applied to data elicited from 17 participants. Results suggest that the diphthongization of these sequences, although constrained by internal linguistic factors, is also shaped by external sociolinguistic factors such as region and style. A comparative analysis showed significant differences between Caribbean and Andean Spanish. While the former tends to neutralize the difference between diphthong and hiatus sequences in favor of diphthongization, the second tends to keep the phonological contrast. In addition, results indicate that the regional differentiation in the articulation of these sequences by coastal and Andean speakers is clearly established only in formal and careful speech; in informal style the difference tends to be neutralized. These findings contribute to our current understanding of the dynamic interplay between internal and external social factors influencing sound change.