Gold nanoparticles (npAu, 5 nm diameter) stabilized by hexanethiol and dodecanethiol do not exhibit in THF upon laser flash excitation (355 nm) any detectable transient in the microsecond timescale. However, using octadecanethiol as npAu ligand, a weak signal attributed to THF solvated electrons was observed after excitation 355 nm. In contrast, irrespective of the alkyl length of the thiol ligand, irradiation of npAu in 1-butyl-3-methylimidazolium ionic liquid allows detecting charge separation in the microsecond time scale. By quenching with methyl viologen (MV) and determining the concentration of MV(+) , it was estimated that the quantum yield of charge separation state in ionic liquid was 0.12.