We present a theory for the transition to convection for flat chemical wave fronts propagating upward. The theory is based on the hydrodynamic equations and the one-variable reaction-diffusion equation that describes the chemical front for the iodate-arsenous acid reaction. The reaction term involves the reaction rate constants and the chemical composition of the mixture. This allows the discussion of the effects of the different chemical variables on the transition to convection. We have studied perturbations of different wavelengths on an unbounded flat chemical front and found that for wavelengths larger than a critical wavelength (λ > λc) the perturbations grow in time, while for smaller wavelengths the perturbations diminish. The critical wavelength depends not only on the density difference between the unreacted and reacted fluids, but also on the speed and thickness of the chemical front. © 1993 American Institute of Physics.