In the present paper, we consider the effect of solute immobilization on a stability of planar diffusion front in gravity field. The case when heavy admixture diffuses from upper boundary into a bulk of semi-infinite domain of porous medium is investigated. In this situation, the base state formed by the diffusion corresponds to the layer of heavier liquid located above the lighter one. This unsteady base state is potentially unstable to the Rayleigh-Taylor fingering instability of front. The instability conditions are examined using the quasi-static method within the fMIM diffusion model. It is found that the immobilization results in the increase in the critical time for instability significantly, whereas the effect of immobilization on the critical wave number is weak. The stability maps in the parameter space are obtained.