The structure and mechanism of J-aggregates of a sensitizing dye formed on AgBr grains in emulsions were investigated on the basis of thermodynamic analysis of the desorption process of dye molecules from the grains by use of a macroporous cross-linked polystyrene resin as an adsorbent for the dye. The desorption rate did not depend on the residual dye concentration on the grain surface, indicating that J-aggregates were linear (or two-dimensional) in structure. The increase in the average size of J-aggregates on the grains caused (1) the decrease in the desorption rate, (2) increase in the activation energy for the desorption, and (3) increase in the heat of adsorption (-Delta H) of the dye to the grains. Thermodynamic analysis of these results supported the previously proposed mechanism, according to which J-aggregates of dye on silver halide grains grew through the repetition of the desorption and adsorption of dye molecules on the grains.