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 ms), using a point-plane electrode arrangement. The fillers the authors investigated are MoS 2 , Al 2 CoO 4 , SiO 2 and CaF 2 . The gases and mixture the authors considered are SF 6 , CO 2 and SF 6 -CO 2 . It is shown that the stopping length of discharges L f increases quasi-linearly with the voltage; L f is shorter in SF 6 than in CO 2 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 L f is obtained with PTFE filled with MoS 2 or Al 2 CoO 4 . Thus, insulators with these fillers appear as the best insulators.