Although there is an increasing number of middle schools establishing English immersion programs, teachers are not well-understood immersion and its application, as educators would hope them to do. Inadequate attention is paid to how teachers teach, how in-service training influences their understanding, and their practice of teaching. There is a pressing need to address these issues. This study is a mixed research design aimed at exploring selected in-service teachers' understanding of English immersion, and their experiences. Data were obtained through a questionnaire, in-depth individual interviews, classroom observation, and post-observation discussion. The findings indicate there was confusion about content teaching "vs." English teaching, and sometimes a failure to understand why teaching content in English was necessitated. The novice teachers have more irregularities between their understanding and practices. Factors, such as insufficiencies of in-service training, lacking time for planning and exposure to English practising activities; and absence of a policy framework and inadequate support, were identified as the cause for less success of the English immersion program. Implications are discussed, and suggestions for future studies are provided.