Résumé : The relative role of convection and diffusion is characterized both numerically andexperimentally for porous media flows due to a Rayleigh-Taylor instability of a horizontalinterface between two miscible solutions in the gravity field. We show that, though globallyconvection dominates over diffusion during the nonlinear regime, diffusion can locally beas important as convection and even dominates over lateral convection far away from thefingertips. Our experimental and numerical computations of the temporal evolution of themixing length, the width of the fingers, and their wavelength are in good agreement andshow that the lateral evolution of fingers is governed by diffusion