Classrooms incorporating Cooperative Learning (CL) structures facilitate a supportive learning environment for English Language Learners (ELLs). Accurate pronunciation by ELLs is important for communication, and also benefits academic achievement. The known benefits of CL for ELLs make it a desirable learning environment to teach pronunciation skills. Thus, sufficient consideration must be given to its incorporation in curricula and delivery format. However, in implementing CL structures in ELL classrooms, attention must be paid to incubation time for CL elements, social and cultural factors. This quasi-experiment, with a pre-post test two-treatment, formed by expanding across program design investigated the impact of a CL environment in the pronunciation skills of ELLs. Results showed that the difference between the improvement of the CL (experimental) group and the control group was not significant enough to claim that CL was a major differential factor. Factors affecting the implementation of CL in ELL classrooms are discussed.