The effect of ionic repulsion among hydrophilic groups of arachidic acid molecules on the aggregation structure of the arachidic acid monolayer on the water surface was investigated on the basis of surface pressure-surface area (pi-A) isotherm measurements, Fourier transform infrared spectroscopic (FT-IR) measurements, and transmission electron microscopic (TEM) observations. pi-A isotherm and FT-IR measurements revealed that the degree of ionic dissociation of hydrophilic groups in the monolayer drastically changed in a pH range of 10-12. The arachidic acid monolayer in a slightly dissociated state of hydrophilic groups (pH = 5.8) did not exhibit the phase transition from an amorphous state to a crystalline one during compression. On the other hand, the monolayer in a highly dissociated state (pH = 12.6) showed the phase transition from an amorphous state to a crystalline one by surface compression at the subphase temperature, T-sp, below the melting temperature, T-m, of the monolayer, and also, the monolayer was still in an amorphous state without the phase transition to a crystalline state at T-sp above T-m. The aggregation structure of the monolayers on the water surface was systematically classified into "the crystalline monolayer", "the amorphous monolayer", and "the compressing crystallized monolayer" with respect to the thermal and chemical (intermolecular repulsive) factors.