A change in the mode of self-motion coupled with complex formation was investigated experimentally and theoretically. A 1,10-phenanthroline disk floating on water exhibited either uniform or intermittent motion (in which the disk altered between rapid motion and a resting state) depending on the concentration of Fe(2+) in the aqueous phase. Since the driving force for this motion is considered to be governed by the surface concentrations of phenanthroline (phen) and [Fe(phen)(3)](2+), it is important to understand the relationship between the kinetics near the air/water interface and the mode of motion. We propose a mathematical model for the motion of the phenanthroline disk and discuss the validity of this model based on a comparison with the experimental results.