An investigation is made of the stability to infinitesimal disturbances of a horizontal layer of dissociating fluid, heated from above or below. A modification of mathematical model due to Lighthill is employed. The results of the linear perturbation analysis of the normal mode resolution of the disturbances show that: (1) there is a slight departure in the onset of convective instability from the classical Be´nard problem for a nonreactive fluid layer heated from below; (2) for the case of heating from above, which is stable for a nonreactive fluid, there is also an onset of instability. Such a flow situation which is stable if the fluid is not chemically reactive but can exhibit an instability if the fluid is chemically reactive (i.e., in this case a dissociating fluid) is referred to as achemical instability.