This study presents a systematic literature review of the monophthongization of the diphthong in Brazilian Portuguese. Monophthongization is a sound change by which a diphthong becomes a single vowel. Thus, the output of, for example, the word beira (‘edge’) can be b[eI ]ra or b[e]ra. Our primary sources, 10 Master’s theses that analyzed this phenomenon using quantitative sociolinguistic methodologies, focus on individually describing a region’s variety of Portuguese. However, the results were never systematically related to each other. Consequently, these works do not present a comprehensive overview of the production of in Brazilian Portuguese. Therefore, this systematic review gathers and unifies information dispersed in these studies, aiming to offer an overview of this optional phenomenon. The overall results demonstrate that the following context was the relevant linguistic variable, while the speaker’s educational level and dialect variation are the relevant non-linguistic variables for the application of the monophthongization rule.