Monolayer behavior and desorption of calcitonin molecules from the air/water interface under constant surface pressure were investigated. The change of area with time for one molecule in the monolayer was recorded under different temperatures and pH values of the subphase, The desorption of calcitonin occurs only in the plateau region and increases with decreased pH and increased temperature. The desorption process was found to be diffusion controlled. Applying the Ter Minassian Saraga model(J. Chem. Phys. 52, 181 (1955)), the kinetic parameters corresponding to the desorption and diffusion were estimated.