The present experimental study describes the mass transfer across the interface between convection cells in a two-layer, salt-stratified system under a horizontal temperature gradient for various parameters. We employed a small conductivity temperature sensor which is designed to measure simultaneously the temperature and concentration of the fluid. The interfacial mass transfer rate in terms of Sherwood number depends on the thermal Rayleigh number and initial buoyancy ratio. In particular, the Sherwood number is found to significantly increase when the initial buoyancy ratio is much less than unity due to interfacial instabilities.