The onset of buoyancy-driven convection in an initially quiescent, horizontal nanoparticles suspension layer heated from above is analyzed theoretically. In this thermally-stable stratified fluid layer, the Soret diffusion can induce buoyancy-driven motion for the case of the negative separation ratio. For the high solutal Rayleigh number, the convective motion sets in during the transient diffusion stage and the onset time of this motion is analyzed by employing the propagation theory. Here the stability criteria are obtained and the predicted critical time to mark the onset of convective motion, T, is compared with available experimental data. It is shown that convective motion can be detected starting from a certain time tau(m) congruent to 4 tau(c). (c) 2006 American Institute of Chemical Engineers.