This paper reports on Moroccan EFL learners’ acquisition of English diphthongs. It uses the Constraint Demotion Algorithm (CDA henceforth) (Tesar & Smolensky, 1998, 2000) to examine the role of cross-linguistic influence on the acquisition of this particular aspect. Ninety-six Moroccan learners participated in the study. They were evenly divided into three academic levels: Baccalaureate, Second Year University, and Master’s. The participants performed an oral fill-in-the-blanks test that elicited their intuition about the various English diphthongs. The main findings of this study are that learners´ prior linguistic knowledge impacts their acquisition of English diphthongs. Learners tend to fluctuate between the French and the Arabic hierarchy before they were able to converge to the English one. Besides, the results suggest an influential role of academic level as learners’ pronunciation errors were shown to diminish progressively as they climb the academic ladder.