Small-angle X-ray scattering, infrared absorption measurements, mechanical testing and ESCA methods were applied for studying the structural and chemical transformations in thin films of statistical multiblock-copolymers based on rigid aromatic oligosulfone blocks and soft oligobutadiene blocks of different relative content of blocks in the course of their treatment with fluorine (F-2) containing gaseous mixtures. The S-shaped rise of polybutadiene microphase density against fluorine content in copolymers was detected for the samples where that microphase was continuous. It was found that enlarged amount of HF molecules can be accumulated within the soft block microphase as a consequence of associated chemical reactions and that HF molecules may form an appropriate ＇＇network＇＇ due to the strong hydrogen bonds formation thus changing dramatically the fluorination kinetics of soft and rigid blocks. The results of this work are useful for preparation of high effective gas mixture separation polymeric membranes by direct fluorine treatment.