I describe and analyze a process of postlabial rounding in A’ingae (isolate, iso 639-3: con): after labial consonants, the diphthong / ae / may surface as [ oe ]. However, a postlabial monophthongal / a / always surfaces faithfully as [ a ]. To capture these facts, I propose an analysis couched in Q-Theory, where one vocalic target of a diphthong corresponds to fewer subsegments than a monophthong. This predicts that diphthongs might show an emergence-of-the-unmarked (TETU) effect, while monophthongs surface faithfully. The prediction is borne out by A’ingae postlabial rounding, contributing a novel argument for the Q-Theoretic representations. Finally, I show that similar asymmetries between diphthongs and monophthongs have recurred throughout the language’s history, further strengthening the proposal.