The O 2 ( 1 Δ)/I 2 mixing process is one of the most important steps in chemical oxygen–iodine laser (COIL). Based on the chemical fluorescence method (CFM), a diagnostic system was set up to image electronically excited fluorescent I 2 (B 3 П 0 ) by means of a high speed camera. An optimized data analysis approach was proposed to analyze the mixing process of supersonic oxygen–iodine parallel streams, employing a set of qualitative and quantitative parameters and a proper percentage boundary threshold of the fluorescence zone. A slit nozzle bank with supersonic parallel streams and a trip tab set for enhancing the mixing process were designed and fabricated. With the diagnostic system and the data analysis approach, the performance of the trip tab set was examined and is demonstrated in this work. With the mixing enhancement, the fluorescence zone area was enlarged 3.75 times. We have studied the mixing process under different flow conditions and demonstrated the mixing properties with different iodine buffer gases, including N 2 , Ar, He and CO 2 . It was found that, among the four tested gases, Ar had the best penetration ability, whilst He showed the best free diffusion ability, and both of them could be well used as the buffer gas in our experiments. These experimental results can be useful for designing and optimizing COIL systems. • An analysis method of O 2 ( 1 Δ)/I 2 mixing process using CFM was proposed. • A trip tab set was demonstrated to enhance O 2 ( 1 Δ)/I 2 mixing efficiently. • The optimal flow conditions for mixing with different buffer gases were given. • A comparison between different gases used as iodine buffer gas was conducted.