The purpose of this study was to assess how the Inquiry Based Learning approach supports the development of students' academic language proficiency in invertebrate systematic. The study was framed by Halliday's sociolinguistic approach to learning. A total of 50 classroom observation sessions at 10 schools were conducted: 10 before and 40 after the intervention. Twenty purposively selected teachers and 200 randomly selected Form 4 students participated in this study. Data were collected using a mixed-method research approach through a lesson observation checklist and evaluation sheet. Further, qualitative data were analysed thematically, and themes were transcribed and coded. Quantitative data were analysed using frequencies and percentages. Key findings revealed a progressive positive change in teaching and learning invertebrates using the 5Es (Engage/Excite, Explore, Explain, Elaborate, Evaluate) instructional model. Students' writing skills were highly improved, and the phases which contributed to the improvement included, in particular, Explore and Explain. However, reading was rarely observed in any phases of the 5Es cycle. The study recommends the use of the inquiry-based learning approach in teaching and learning of invertebrate systematic to improve students' academic language proficiency at secondary school level.