In this study, we applied real-time holographic interferometry to double-diffusive convection in a two-layer, solutally stratified aqueous solution of Na2CO3 destabilized by, lateral heating and cooling and attempted to calculate the value of the local concentrations from interference fringes obtained in this system. To this end, we employed the facts that interference fringes coincide with contour lines of reflective index. and that both temperature and concentration influence reflective index. As a result, local concentrations at given times in a quasi-steady state were determined using interference fringes and measured local temperatures. The concentration calculated from interference fringes agreed with those measured by sampling of local solution. This quantitative agreement demonstrated the reliability of the results.