Adsorption of the nonionic surfactant Triton X-100 at the interface between two immiscible electrolyte solutions (ITIES) and its effect on the electron transfer (ET) reaction between tetracyanoquinodimethane in 1,2-dichloethane and aqueous Fe(CN)(6)(4-) was studied using microelectrochemical measurements at expanding droplets (MEMED). A numerical model was developed for this process by assuming that adsorption of the surfactant at the ITIES was Langmuirian and that the ET reaction only occurred at the uncovered portion of the ITIES. Theoretical results show that, for typical MEMED conditions, the surfactant adsorption. process attains the diffusion-controlled limit if the rate constant is' greater than 1 cm s(-1). The inhibitory effect of surfactant adsorption on the ET process produces changes in the reactant concentration profile adjacent to the droplet, which depend on the bulk surfactant concentration, equilibrium constant (K), the maximum surface coverage (Gamma (max)), and the ET kinetics. Methods for determining these parameters are suggested. The effect of Triton X-100 on the ET reaction was measured over a; wide range of conditions, with bulk aqueous Triton X-100 concentrations in the range 2.5 x 10(-5) to 2.5 x 10(-4) M, over various time scales. Experimental results were found to be in excellent agreement with theoretical predictions and yielded an ET rate constant of 0.0020 +/- 0.0001 cm s(-1) for the clean interface, with the potential across the ITIES established with 0.1 M ClO4- in each phase. The diminution in the ET rate with surfactant present was consistent with diffusion-controlled surfactant adsorption, characterized by K = 2.7 x 10(4) M-1 and a value of Gamma (max) = 3 x 10(-10) mol cm(-2).