Rock weathering is driven by reactions and transport mediated by water, which exists on the surface and the interior of a rock. Water saturation in a rock varies depending on the conditions of rainfall and drying, resulting in complex distributions of water and air. As the rock matrix proceeds to dry, pore water is lost first from larger pores and then from smaller ones, although evaporation occurs at any pore size. This pore-size dependence of water loss occurs because water in larger pores migrates into smaller pores due to the difference in capillary pressure between the pores. When water infiltrates into dry pores, water preferentially advances into pores having smaller apertures. In addition, air is preferentially entrapped in pores of a specific size. This entrapped air blocks the water flow and significantly decreases the hydraulic conductivity of the rock. The pore wall of entrapped air is covered with a water film having a thickness on a nanometer scale. The thickness of the water film is mainly controlled by pore size and mineral composition if pore water is dilute. From the perspective of the role of the distribution of water on rock weathering, it is important to advance knowledge of the characteristics of air entrapment and water film.