Spatiotemporal pattern formation in the autocatalytic chlorite-tetrathionate reaction is studied experimentally in porous media where heterogeneities are introduced as glass beads packed in a monolayer. From initially vertical planar chemical fronts in a vertical slab, asymmetric convective structures with stable geometry evolve which propagate horizontally with constant speed. The single convection roll is characterized by its mixing length which scales with both the solution height and the permeability of the system. (C) 2012 Elsevier B.V. All rights reserved.