The solubility of benzothiazolium ionic liquids in water and in furfural was determined by static analytical and dynamic methods, respectively. The solidliquid equilibrium data were correlated by using the nonrandom two-liquid (NRTL), Wilson, modified Apelblat, and lambda h models, respectively. Among the models, the modified Apelblat gave the best correlation results with the average relative deviation (ARD) of 2.02%. The effect of alkyl chain length on the solubility was discussed. The (liquid + liquid) phase equilibrium was observed in the system of benzothiazolium ionic liquids and water below the melting temperature of the ionic liquid. The solubility of benzothiazolium ionic liquids in furfural was much higher than in water, indicating the potential of application in extraction of furfural from aqueous solution.