When a reactive fluid circulates inside a porous medium it can dissolve some minerals if equilibrium is not reached and modify the porosity and permeability. The positive feedback between fluid transport and mineral dissolution lead to complex reaction front morphologies such as fingers. Our study is carried out with two objectives: 1) to evaluate experimentally these processes at a decimeter scale, 2) to compare the experiment to a numerical model of water-rock interaction. The experiment consists of a two-dimensional porous medium that allows for the dissolution of halite under an imposed fluid flow. The numerical code used solves the equations of reaction and transport in a porous medium. Both experiment and numerical simulation indicate the development of an instability whose propagation rate depends on the rate of water injection raised to a 2/3 power. ¿ 1998 American Geophysical Union