Over the last few years, ionic liquids (ILs) have emerged as an important class of reaction media for the synthesis of nanoparticles. The formation and stabilization of nanoparticles was investigated in different ILs to elucidate the effect of the chemical nature of the IL anion, cation and alkyl side chain of the imidazolium. In this context, Co-2(CO)(8) was employed as a precursor and thermally decomposed to the metallic cobalt nanoparticles in a series of selected ILs, where either the IL cation or anion was varied while keeping all of the other parameters constant. The results show that both the molecular volume of the anion and cation and the steric configuration are important aspects to control the formation and stability of nanoparticles in ILs.