The onset time of double-diffusive convection in time-dependent, nonlinear concentration fields was investigated theoretically and experimentally. The stability analysis was conducted on the basis of the propagation theory. Under the linear stability theory, the linearized perturbation equations were transformed similarly by using the concentration penetration depth as a length-scaling factor. The newly derived stability equations were solved numerically. Also the onset time was determined experimentally by employing an electrochemical technique where 0.03-0.3 M CuSO4 + 1.5 M H2SO4 aqueous solutions were adopted as electrolyte. The onset time of double-diffusive convection was delayed or shortened depending on the degree of stable stratification. (c) 2005 Elsevier Ltd. All rights reserved.