Instabilities often arise in the sedimentation of colloidal particles, even when using the density gradient technique. These instabilities occur at volume fractions as low as 10(-3) or 10(-4), causing mixing of particles throughout the suspension, rather than the smooth sedimentation of particles. Here, we show that the mixing is due to a classical Rayleigh-Benard instability. The sedimentation process is modeled in an approximate manner, and experiments are done to test whether the approximate model gives maximum stable volume fractions in the correct range. Sedimentation is a well-studied problem, and yet, to our knowledge, this is the first time that the well-known instability has been described in terms of a Rayleigh-Benard instability.