The purpose of this paper is to assess the coupled effect of fluid flow and dissolution/precipitation reactions in heterogeneous porous media at semi-macroscopic scales (i.e. several tens of grains). Are the dissolution/precipitation processes roughening or smoothing at these scales? Does self-organization arise from the coupling of these processes with fluid flow? What is the effect of heterogeneity (i.e. of the variance of the pore size distribution assuming a constant mean)? The dissolution/precipitation processes were modeled as producing a change of the local pore radius proportionally to the local flux. It was found that both precipitation and dissolution lead to more heterogeneous, anisotropic pore structures (parallel to the macroscopic flow for dissolution, perpendicular for precipitation). One new interesting observation was that precipitation generated an almost uniform flux field. To a lesser extent, dissolution also tended to smooth the pressure gradient field. The possibility of uniform flux or pressure gradient fields could have important implications in hydrology or in technical applications such as filter plugging or acid treatment of oil reservoirs. ¿ American Geophysical Union 1996