Spontaneous oscillation of interfacial concentration was calculated based on a partial differential equation with single state variable. Model of interface used was as follows. Homogeneous distribution of molecules adsorbed at interface was thermodynamically unstable in a certain concentration range and spinodal decomposition proceeded. Adsorption/desorption kinetics were expressed by a nonlinear function which had a single stable point. The single point was in thermodynamically unstable region of interfacial free energy. Then, periodic cycle of interfacial concentration appeared. Patterns obtained by present model were similar to those reported in previous papers for electrochemical oscillations at oil/water interface. (C) 2002 Elsevier Science B.V. All rights reserved.