A model was developed to reflect the effect of mass transfer resistance in the gas phase on the net process rate and the diffusion of ozone and organic compound accompanied by simultaneous complex chemical reaction in the liquid phase. Simulations were presented involving various operating conditions of ozonization of aqueous formic acid in a countercurrent gas-liquid absorber with the gas plug flow and the dispersed liquid flow. Ozonization selectivity was presented in relation to temperature and pH of the HCOOH solution. The model is applicable to ozonization of C6H6. PhOH, PAHs, etc.