Diphthong Pronunciation Errors among Final-Year Congolese EFL Learners: A Comparative Study of Public and Private Secondary Schools in Boma, DRC

Pronunciation, and diphthongs in particular, remains a persistent obstacle to oral proficiency for English as a Foreign Language (EFL) learners whose first languages lack equivalent vowel glides, yet Congolese EFL contexts remain largely absent from the empirical literature on this problem. This study investigated English diphthong pronunciation errors produced by final-year EFL learners in public and private secondary schools at Boma, Kongo Central, in the Democratic Republic of the Congo (DRC). A descriptive-comparative design was adopted with 200 final-year learners (n = 100 public, n = 100 private) from 20 secondary schools; each learner completed one of three reading tasks (word list, sentence list, or paragraph) sampling all eight English diphthongs, with errors scored perceptually on a 10-token scale, and a questionnaire on pronunciation background. Results showed that the closing diphthongs /eɪ/, /aɪ/, and /əʊ/, together with the centering diphthongs /ɛə/ and /ʊə/, were the most problematic, chiefly through monophthongization and vowel substitution. An independent-samples t-test comparing public (M = 5.85, SD = 1.18) and private (M = 6.11, SD = 1.18) school learners revealed no statistically significant difference, t(198) = -1.56, p = .121, and the null hypothesis was retained. Questionnaire data implicated mother-tongue interference, limited oral practice, and insufficient explicit pronunciation instruction. These findings suggest diphthong difficulty is a shared, network-independent phenomenon rooted in first-language transfer and pedagogical gaps, with implications for pronunciation-focused teacher training and curriculum reform in the DRC.