Evaluating qualities of English teachers in commercial English language centres: The development of a scale

English teacher quality seems to correlate with the English learners' achievement of learning outcomes; however, there is still a lack of studies into the qualities of English teachers who work at commercial English language centres (CEC). This article reports the process of developing a scale that can be used to evaluate the qualities of English teachers working for CEC in Vietnam. The scale was developed in two stages. In the first stage, the researcher tested content validity of the scale fundamentally using the Delphi technique with the support of eight experts in English education. In the second stage, construct validity and reliability of the scale were tested with Cronbach's coefficient alpha, item-total correlations and factor analysis with varimax rotation. The resulting scale with three principal components - (i) knowledge and English competence, (ii) pedagogical practice and (iii) professional and personal attributes - appeared consistent and valid. Data obtained from using the scale could give implications for the recruitment and professional development of English teachers in CEC in Vietnam. However, sharing the steps and challenges faced during the process of developing the scale would be more important for those interested in developing a similar evaluation tool in a CEC context beyond Vietnam.