He-Ne laser holographic interferometry is applied to double-diffusive convection in Na2CO3 aqueous solutions stratified in two layers when destabilized by lateral heating and cooling. Based on knowledge cultivated in the past, a summary of the density field is conjectured by the temporal changes of interference fringes. Because thermal and solutal gradients coexist, reflective index contours do not correspond to density contours. However, it is found that interference fringes in this system can be considered as density contours except for the circumference of the layers. Variations of density distribution going with interfacial mass transfer are explained in detail.