In the simplified case considered, particles decrease the critical temperature difference for the onset of convection by increasing the heat capacity of the fluid. A proof of exchange of stabilities shows that there is no effect of particle drag when instability sets in, although the rate at which unstable perturbations grow is affected by this factor. Thermal force alone can, in principle, give rise to instability in layers heated from above or below. The results are discussed with reference to experiments in gas‐particle mixtures which show “columnar instabilities” at unusually low‐temperature differences.