The effect and mechanism of the hydrophilic ionic liquids (ILs) with cobalt naphthenate as catalyst on the liquid-phase oxidation of toluene were studied systematically. Five ILs with different hydrophilicities were employed as reaction media. The results showed that toluene conversion increased with increasing hydrophilicity of ILs. The toluene conversion and the corresponding selectivity of benzaldehyde reached 19.6% and 19.5%, respectively, in 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim]BF4). The solubilities of toluene, benzaldehyde, benzyl alcohol and benzoic acid in [Emim]BF4 were further measured at temperatures from 299.2 K to 343.2 K. The correlations between solubility and temperature were developed based on the critical point of phase separation or ideal solution model. The low solubility of toluene in [Emim]BF4 indicated a heterogeneous oxidation mechanism for toluene. The improvement of toluene conversion and product selectivity in [Emim]BF4 was attributed to the interactions between reaction and extraction separation. The sensitivity study demonstrated that the increase in oxygen pressure was beneficial to the reaction. A much lower reaction temperature of 130 degrees C was achieved in [Emim]BF4 instead of a typical 160-170 degrees C for current commercial plants. The high conversion, low reaction temperature and high product selectivity for the liquid-phase oxidation of toluene in [Emim]BF4 makes it a green and efficient hydrocarbon oxidation process. (C) 2012 Elsevier B.V. All rights reserved.