Numerical simulations of contaminated bubbles or drops adopt a model of adsorption-desorption kinetics developed for quiescent systems. However, the model has rarely been validated due to the lack of experimental data of surfactant concentration at a moving interface. Hence, we experimentally investigated surfactant concentration at the moving interface of a spherical drop falling in a stagnant liquid containing surfactant using the velocity distributions measured by spatiotemporal filter velocimetry (SFV). The molar flux of surfactant to the interface was also evaluated by substituting the measured velocity and surfactant concentration into the conservation equation of surface concentration of surfactant to examine the applicability of the Frumkin–Levich model to a drop falling in a contaminated system. We confirmed that the evaluation of surfactant concentration using SFV is of great use in understanding adsorption-desorption kinetics at an interface in a contaminated system and for validation of adsorption and desorption models. The measured results showed that the Frumkin–Levich model is not applicable to a moving interface, whereas it is applicable to an immobile interface, that the dependence of the molar flux on the molecular weight of surfactant is not so strong, and that the ratio of the surface concentration of surfactant to the maximum concentration, i.e. the surface coverage, is an appropriate index for judging an applicable range of the Frumkin–Levich model.