Dynamic surface pressure (pi)-surface area (A) characteristics were measured using periodic changes in the surface area of an unsaturated phospholipid (1,2-dioleoyl-3-sn-phosphatidylcholine; DOPC) thin film formed at an air/water interface. The dynamic pi-A curve traced a single closed hysteresis loop after several cycles of compression and expansion, and the shape of the loop depended on the frequency of the change in surface area, the subphase temperature, and the chemical species dissolved in the aqueous subphase. To evaluate the loop shape in a quantitative manner, we carried out Fourier transformation of the time-dependent change in surface pressure. On the basis of the higher harmonic components in the Fourier spectra, it is clear that a DOPC thin film behaves as a nonlinear viscoelastic body during compression and expansion. We established a useful kinetic model by taking into account the cooperative aggregation/dissociation of DOPC domains at the air/water interface with a periodic change in surface area. Hysteresis loops were examined with the aid of a computer simulation which included the cooperative factor.