Density fingering in the chlorite-tetrathionate reaction has been studied experimentally in Hele-Shaw cells with spatially modulated gapwidth. The periodic modulation parallel and perpendicular to the front propagation is introduced in the form of round depressions in the inner wall of the reaction vessel. Both the initial and the long time behavior have been studied while varying the chemical composition, the gapwidth, and the periodicity of the modulation. We have shown that although the initial pattern formation is unaffected, an enhanced tip splitting accompanies the general coarsening of the structures during the long time behavior. (C) 2009 Elsevier B. V. All rights reserved.