Highly concentrated hydrosols of silica acid were applied to porous Pinczów limestone and Nietulisko sandstone. Silicic acid sols were obtained from water glass R140 by means of ion exchange with a calcionite (Amberlit IRC50). Results show that the rate of sol rise in limestone is at its lowest, the higher its concentration while in porous sandstone they may be applied without limitation. The distribution of the consolidant in the limestone depends on its concentration, its pH, and the addition of a catalyst. The best results were noticed for pH 10 sols obtained from a 15 or 20% solution of water glass. An addition of MgCl₂ causes an important increase in viscosity. Structural analyses of the consolidated sandstone by scanning electron microscopy show precipitated silica to be amorphous and distributed homogeneously in the pores. The applied sols improve mechanical resistance of the limestone, and also the stone resistance to freeze-thaw effects and the attack of water-soluble salts.