This study is the first comprehensive work in the literature to investigate service quality dimensions in the contexts of retail/business customers of traditional and Islamic banks. The first phase of the study explored concepts, dimensions and models of service quality. The hypotheses were developed in the second phase. The third phase was concerned with the empirical work. The findings are firstly- technical quality, functional quality and image together have been found to be better fit than SERVQUAL alone as predictors of perceived service quality. The results also indicate that functional quality is more important than technical quality. Secondly, the type of customer (retail/business) was found to have a significant impact on the relative importance of some aspects of functional quality. Thirdly, the research reveals only a few differences between the retail and business customers of banks, and a few differences between Islamic and non-Islamic bank customers in terms of the relative importance of service quality dimensions. Practically, the study provides a well developed measure of technical quality and a series of useful recommendations for banks’ managers.