Homogeneous exoplasmic leaflets of biomembrane are mimicked by lipid monolayers at the air/water interface. The reaction rates of lipase (Pseudomonas cepacia) catalyzed hydrolysis of a substrate (umbelliferone stearate) on L-alpha-dilauroylphosphatidylcholine/cholesterol mixed monolayers at the air/water interface are examined as a function of cholesterol composition, which is to vary the dynamics of the system. Lateral mobility as a measure of dynamics of phospholipids and adsorbed lipase molecules are probed with the technique of fluorescence recovery after photobleaching. Upon correlating the lateral diffusion coefficients of a probe lipid and lipase with the interfacial hydrolysis kinetics, we show for the first time that the catalytic reactions on the monolayers are diffusion-controlled. Moreover. our results are in a quantitative agreement with the two-dimensional reaction dynamics theory of Torney and McConnell.