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This study is devoted to the optical and electrical characterisation of discharges propagating over insulators made of polytetrafluoroethylene (PTFE) filled with different kinds of micro-mineral fillers immersed in gas or gaseous mixture, under lightning impulse voltage (1.2/50 s), using a point-plane electrode arrangement. The fillers the authors investigated are MoS2, Al2CoO4, SiO2 and CaF2. The gases and mixture the authors considered are SF6, CO2 and SF6-CO2. It is shown that the stopping length of discharges Lf increases quasi-linearly with the voltage; Lf is shorter in SF6 than in CO2 and it is higher when the point electrode is positive than when it is negative while the initiation voltage of discharges is higher with a negative point than with a positive one. The discharges do not always present a radial structure as reported in the literature. Also, the type of filler greatly influences the characteristics of creeping discharges. In a given gas or mixture, the shortest Lf is obtained with PTFE filled with MoS2 or Al2CoO4. Thus, insulators with these fillers appear as the best insulators.