English-medium Instruction (EMI) is increasingly being implemented in diverse educational settings where English is not the primary language. However, a persistent challenge of EMI implementation lies in the scarcity of high-quality EMI teaching materials that effectively enhance both content knowledge and language acquisition. This study aimed to investigate how language is integrated with content in EMI lessons and to assess their efficacy. The study involved three Civil Engineering teachers, three language teachers, and 102 students. Data collection instruments comprised journals kept by content and language teachers during the EMI materials development process, pre-tests and post-tests, satisfaction surveys, and semi-structured interviews. The quantitative results were analyzed using descriptive statistics and t-tests, while content analysis was used to analyze qualitative data. Results revealed that students expressed high satisfaction with the materials, noting increased participation in EMI classes, enhanced content comprehension, and heightened awareness of language use in lessons. Nonetheless, the design process of EMI materials is intricate, non-linear, time-consuming, and demanding of pedagogical expertise. While collaborative efforts between content and language teachers have appeared promising in theory, practical implementation in real contexts has proven to be challenging. It is recommended that the EMI materials developers possess pedagogical knowledge on content and language teaching, recognize the iterative and complex nature of the material design process, and the need for ongoing communication with the respective to enhance the success of the collaboration.