The kinetics of diffusion and adsorption of a water-soluble nonionic surfactant, polyoxyethylene sorbitan monolaurate (Tween-20), to the interface between two viscous immiscible liquids relevant to many applications is experimentally investigated using drop volume tensiometry. The measured variation in dynamic interfacial tension with contact time at different bulk concentrations of the surfactant in the viscous aqueous phase indicates that the equilibrium is slowly attained at low concentrations. As the bulk surfactant concentration is increased by two orders of magnitude, the contact time needed to reach equilibrium decreased by about an order of magnitude. The corresponding values of diffusion coefficient in the viscous liquid, determined using existing theoretical equations, are found to be almost constant. These are about two orders of magnitude smaller than those for the data available in literature for similar surfactants in much less viscous water at the air interface. The molecular area of Tween-20 determined in the present work is found to be very close to the value reported by other authors.