Molecular level characterization of dissolved organic sulfur (DOS) by electrospray ionizationFourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR MS) is necessary for further understanding of the role of DOS in the environment.Here,ESI spray solvent,a key parameter for ion production during ESI process,was investigated for its effect on the molecular characterization of DOS by ESI-FTICR MS.100％ MeOH as spray solvent was found for the first time to remarkably enhance the ionization efficiency of the majority of CHOS-molecules in NOM,which facilitated a total of 1473 CHOS-molecular formulas with one sulfur atom to be detected.The number of CHOS-molecular formulas obtained using 100％MeOH as spray solvent increased notably over 740 in comparison with those using 50％ MeOH aqueous solution (731) or 50％ ACN aqueous solution (653).Moreover,due to the enhancement of ionization efficiency of DOS during ESI processes,the tandem mass spectra of the NOM CHOS-molecules could be easily obtained using 100％ MeOH as spray solvent,which were hardly obtained using 50％ MeOH aqueous solution as spray solvent.The results of the tandem mass spectra suggested the first discovery of organosulfatas or sulfonic acids in Suwannee River NOM sample.A simple method based on 100％ MeOH as ESI spray solvent for advanced molecular characterization of DOS by ESI-FTICR MS was proposed and applied,and the results revealed more molecular information of DOS in sea DOM samples.