The effect of the solution viscosity on the rate of tetraethylammonium ion (TEA(+)) transfer across the nitrobenzene/water interface has been studied by adding sucrose in the aqueous phase or replacing the aqueous phase with a corresponding heavy-water solution. The addition of sucrose into the aqueous phase up to 40 wt.% does not lower the value of the apparent standard rate constant of TEA(+) ion transfer, which is measured with phase-selective a.c. polarography. The substitution of H2O with D2O also introduces no significant deceleration of the rate of TEA(+) ion transfer. The observed insensitiveness of the rate of ion transfer to the change in bulk viscosity suggests that the thickness of the interfacial layer determining the rate of ion transfer is much less than the order of microns. The interfacial viscosity is not likely to be influenced by the increase in the viscosity of the bulk phase. No sign of the specific adsorption of sucrose is discernible in the differential capacitance.