A two-layer, salt-stratified system destabilized and mixed by lateral heating and cooling is studied experimentally using three different enclosures. The following conditions are considered: thermal Rayleigh number Ra-T = 2 x 10(7)-7 x 10(8) and initial buoyancy ratio N-i = 0.441-0.882. Kelvin-Helmholtz vortices occur, in the initial stages before mixing, within the interface that separates the top and bottom convective layers due to shear instabilities. After that, the interface is significantly tilted and is rolled up near the hot and cold walls, while following the downward migration of the interface at the cold wall, the fluid in the bottom layer penetrates into the top layer along the hot wall, indicating the onset of mixing. These mixing processes depend on the thermal Rayleigh number and initial buoyancy ratio. Finally we propose a correlation for the dimensionless mixing time, valid for initial buoyancy ratios less than unity.