Two processes for the mass transport of C12E4 in a quiescent surfactant solution were studied: (i) the desorption of C12E4 out Of a suddenly compressed interface; (ii) the adsorption of C12E4 Onto a suddenly expanded air-water interface. A video-enhanced pendant bubble tensiometer was used for the measurement of surface tension relaxation due to the adsorption or desorption of surfactant molecules. Both processes were found to be diffusion-controlled at dilute concentration but diffusive-kinetic mixed-controlled at more elevated bulk concentration for C12E4 Rate constants of adsorption/desorption were evaluated from the comparison between the surface tension profiles and the theoretical profiles predicted from the Frumkin model. The values of the kinetic rate constants of C12E4 Obtained from these two experiments are nearly the same. A theoretical simulation was also performed to confirm the shifting mechanism (J. Chem. Eng. Jpn. 1996, 29, 634) for C12E4 for both desorption and expansion processes.