Double-diffusive convection in a solutally stratified, two-layered solution destabilized by lateral heating and cooling is studied numerically. A simulation was carried out for Pr = 6.67, Le = 184, Ra-T = 2.43 x 10(7), Ra-c = 3.13 x 10(7) and A = 1.1. Experiments were also performed with aqueous solutions under the same condition. Time evolutions of flow and temperature fields obtained numerically show qualitative agreement with that obtained by the visualization of flow and temperature. Moreover, numerical results of the molar flux of solvent through the interface agree very closely with experimental ones. Thus the numerical algorithm employed here is proved to be appropriate for the analysis of double-diffusive convection. Using the numerical results, the mechanism of mass transfer, which is usually hard to investigate through experiments, is discussed. The double-diffusion process until the two layers start mixing can be divided into three periods; (A) a period until a thin interface is formed between the two layers, (B) a period during which the temperature field is in quasi-steady state, and (C) a period when the interface between the two layers is unstable. Numerical results show that each period has different mechanism of mass transfer.