Controlling the adsorption behavior of bulk-phase self-assembling dye molecules at solid/liquid interfaces is of importance for application to various devices. Herein, we report an unexpected phenomenon on the adsorption behaviors of bulk J-aggregating water-soluble porphyrin diacids. A comparative study on the adsorption amounts of J-aggregated meso-tetrakis(4-sulfonatophenyl)porphyrin diacid from freshly prepared and pre-aged solutions revealed enhanced adsorption through the self-assembly process (EASAP). The aggregate structure formed by EASAP is almost identical to the one from preformed J-aggregate solutions. The generation ratio of J-aggregates at an interface and in bulk strongly depends on the interface-to-volume ratio of the solutions. The surface property of cuvettes and coexisting inorganic ions has no significant effects on EASAP. While EASAP occurs in the J-aggregations of the other water-soluble porphyrin diacids, it is suggested that self-assembly properties play an important role in the adsorption proportion. These results will provide new insight into the adsorption equilibrium of bulk self-assembling molecules at solid/liquid interfaces