The coupling of molecular diffusion and chemical autocatalysis leads to propagating chemical fronts. The specific type of autocatalysis determines the thickness and speed of the front. Chemical fronts in liquids separate reacted from unreacted fluids with a corresponding mass density gradient. These differences may lead to convective flow which enhances the speed and determines the curvature of the front. The transition to convection is determined not only by the spatial geometry and density differences, but also by the type of chemical reaction. We determine the transition to convection for chemical fronts with different autocatalytic reactions of different order. We study fronts propagating in porous media, in viscous fluids, and fluids confined in a vertical slab. We compare these results with the results based on a thin front approximation using an eikonal relation. (C) 2003 American Institute of Physics.