In order to obtain good biosensor characteristics, an investigation on the mixed octadecylamine/ butyrylcholinesterase monolayer at the air-water interface is conducted. The kinetics of the adsorption of the enzyme at the air-water interface is dependent on time, concentration, and the ionic interaction with the amphiphilic polar groups. From the surface pressure-molecular area isotherms and their stabilities, a model of the mixed film at the air-water interface is proposed and experimental conditions leading to a stable film are defined and explained.