Chemical reactions on a cell membrane surface are of pivotal importance for cellular functions such as intracellular signal transduction. Of the variety of membrane proteins, those imbedded in hemi-leaflets of bilayer membranes can be mimicked structurally and dynamically on monomolecular layers of phospholipids at the air/water interface. This is to report the kinetics of lipase (Pseudomonas cepacia) catalyzed hydrolysis of a substrate (umbelliferone stearate) on uniphasic L-alpha-dilauroylphosphatidylcholine monolayers at the air/water interface. A novel experimental protocol of the interfacial reaction is employed to probe the enzymatic kinetics. The kinetic rate constants on the monolayers are extracted from the substrate and the enzyme concentration dependences of the initial hydrolysis rate. The lipase on monolayers at the air/water interface is highly activated over that in bulk solution and the turnover number of the lipase catalysis is strongly dependent on the surface pressure of monolayers.