A mathematical formulation is derived that describes the transport and chemistry of atmospheric gases as they diffuse through the open stomata, inner air spaces, and cell walls of plant leaves before depositing on the mesophyllic plasmalemma. The formulation is applied to the problem of ozone deposition on plant leaves to determine if reactions in the mesophyllic cell-wall water can prevent ozone from reaching the plasmalemma and initiating harmful oxidation reactions. Calculations indicate that a major portion of the ozone diffusing through the leaf can react with ascorbic acid in the cell wall. Other mechanisms such as ozone reactions with biogenic olefins and ozone decomposition in the cell-wall water are found to be unimportant. These calculations suggest that plants may protect themselves from the harmful effects of ozone by concentrating ascorbic acid in their cell walls; by reacting with ozone in the cell-wall, the ascorbic acid limits the amount of ozone that can penetrate through the cell wall and reach the more vulnerable material inside the wall. © 1989, American Chemical Society. All rights reserved.