In this study, the surface interactions between different dyes and amorphous WO₃ nanosheets in water were investigated. The amorphous WO₃ nanosheets had a strong adsorption for cationic dyes and could be self-precipitated from water after enough dye molecules were absorbed into their micro-structures. The absorbed cationic dyes eventually formed H-aggregates or J-aggregates on the surface of WO₃. In contrast, the amorphous WO₃ nanosheets had a weak or no adsorption capacity for anionic dyes. This selective adsorption property of WO₃ nanosheets could be applied to separate cationic dyes from a mixed solution. The possibilities of pH-induced desorption for cationic dyes on the WO₃ nanosheet surfaces have also been demonstrated in this study, and its mechanism has been discussed in detail. The findings leaded to a better understanding of the dye adsorption behavior on amorphous tungsten oxide surfaces.