Observing the phase separation of deeply quenched, low viscosity liquid mixtures we inferred that the process is driven by the convection due to capillary forces, and not by molecular diffusion neither by gravity, heat or surface effects. After quenching a partially miscible, initially homogeneous, off-critical liquid mixture to a temperature T deeply below its critical point of miscibility T-c, with parallel toT-T(c)parallel to/T(c)approximate to0.1, we observed the formation of rapidly coalescing droplets of the minority phase, whose size grows linearly with time. Following the motion of isolated 10 mum droplets, we saw that they move in random directions at speeds exceeding 100 mum/s, showing that during most of the process the system is far from local equilibrium. Eventually, when their size reaches the capillary length, the nucleating drops start sedimenting as gravity becomes the dominant force. This behavior was observed for both density-segregated and density-matched systems, irrespectively whether they were kept in horizontal or vertical cells. The experiments were repeated using both untreated (i.e., hydrophilic) and modified (i.e., hydrophobic) cell walls, with identical results and, in addition, no bulk motion was observed when the mixture was replaced with water, showing that the observed convection is not induced by gravity, neither by surface or temperature effects. Using a simple dimensional analysis of the governing equations based on the diffuse interface model, we showed that convection is induced by the coalescence among drops which, in turn, is the result of a nonequilibrium capillary force that indeed dominates both diffusion and gravity forces. (C) 2003 American Institute of Physics.