Acetonitrile (ACN) is widely used as solvent for electrolyte in supercapacitors in the presence of tetraethylammonium tetrafluoroborate (Et4N+BF4 −). The main advantage is its low viscosity leading to high conductivities in the presence of salts, but unfortunately it is too volatile and flammable. In order to reduce both the flammability and the volatility of ACN based electrolytes, aprotic room temperature ionic liquids, RTILs, are added either to ACN or to γ-butyrolactone (GBL), another typical solvent for the electrochemical devices. The effects of these molten salts on ACN and GBL are investigated with the aim of improving the supercapacitors security. Flammability tests are performed on these electrolytes to study their security. The results show that mixtures GBL based are more flammable than those ACN based, in spite of a higher boiling point for pure GBL because of the presence of solvates. DSC measurements over ambient temperature strengthen these flammability results. To complete this study, physical and electrochemical characteristics (density, viscosity, conductivity, linear and cyclic voltamperometry) are investigated. The results indicate that no degradation of the electrochemical performances appears with RTIL/ACN mixture by comparison with the standard electrolyte (ACN+1M Et4N+BF4 −).