In the present research, theoretical and experimental studies are conducted to investigate the characteristics of double-diffusive mass transfer in a horizontal fluid layer with thermal stratification. As the double-diffusive convection may either enhance or reduce transfer rates, the present study developed a new predictive model for the transfer correlation of double-diffusive convection analytically. Using an electrochemical system as a double-diffusive environment, the double-diffusive conditions to mark steady ionic mass transfer were investigated experimentally. To do this, the limiting current technique was adopted in a CuSO₄H₂SO₄ solution confined within horizontal copper electrodes. Theoretical results were confirmed by the experiments in which the relations of the double-diffusive conditions were quantified in an electrolyte-horizontal layer with both electrostatic fields and temperature stratifications. Thermal stratification showed the stabilizing effects in a multicomponent fluid layer as reducing the magnitude of buoyancy forces. The present study provides plausible predictions and explanations in controlling mass and heat transfer rates for various practical situations including the double-diffusive convection.