To reveal the impact of ionic liquids (ILs) on the stability of proteins, a series of ILs possessing same 1-butyl-3-methylimidazolium cation [Bmim](+) with a set of Hofmeister anions such as SCN-, HSO4-, Cl-, Br-, CH3COO- and I- were used and their effects on the myoglobin (Mb) structure and stability were studied. For the sake of comparison and also to explore the extent of the stabilization behavior of ILs toward Mb stability, we have chosen a set of ionic salts (I-s) of a fixed sodium cation (Na+) with the same series of anions such as SCN-, SO4-2, Cl-, Br-, CH3COO- and I-. UV-vis, fluorescence and circular dichroism (CD) spectroscopic techniques were used in order to investigate the stability behavior of Mb in ionic species (I-s and ILs). The results reveal that both I-s and ILs had a negative influence on the stability of Mb. Apparently, the flexibility in the native structure of Mb gradually increases with the increase in the concentration of I-s and ILs at pH 7.0. Therefore, a sharp decrease in the transition temperature (T-m) of the native Mb is observed in the presence of I-s and ILs. (C) 2014 Elsevier Ltd. All rights reserved.