BackgroundAt the State Europe School of Berlin (SESB) students with different language backgrounds learn together in two languages of instruction: German and one of nine partner languages (English, French, Greek, Italian, Polish, Portuguese, Russian, Spanish, and Turkish). AimsThis study investigates the reading proficiency trajectories in the minority or partner language among students in a two-way immersion (TWI) program. SampleParticipants were 977 TWI students. MethodsThe study included longitudinal large-scale assessments in Grades 4 to 6. Latent growth curve models were utilized for analysis. ResultsThe analyses revealed significant progress in partner language reading skills across all language combinations. However, notable differences in initial proficiency levels and developmental trajectories were observed between partner languages, even after controlling for background variables such as socioeconomic status and cognitive abilities. Furthermore, the results demonstrate that students' initial German reading proficiency positively influenced their reading proficiency in the partner language. ConclusionsOverall, the study demonstrates the effectiveness of the TWI program in promoting partner language proficiency development in TWI programs, particularly within the unique context of a diverse linguistic environment. This research provides valuable insights into how students develop reading skills in a multilingual environment.