The solvent properties of ZnCl₂ molten salt medium and its synergic effect with the Lewis acid catalyst, Sn⁴⁺, for biomass conversion, were investigated by nuclear magnetic resonance. The tautomeric distribution of fructose in the ZnCl₂ molten salt medium was examined, and its effect for humins formation during the biomass conversion was evaluated. The ion complex composed by Sn⁴⁺ and Zn²⁺ indicated that there is a synergic catalytic effect between these two Lewis acid ions. ¹³C NMR spectra of fructose in different ZnCl₂ molten salt hydrate concentrations revealed that the concentration of β-furanose and α-furanose tautomers, which lead to 5-HMF, were significantly decreased with increased salt concentration. Meanwhile the β-pyranose tautomer, which is correlated with humins formation, was increased significantly.