Investigations have shown the porous structure of stone affects its durability. The most harmful parameter of the porous structure is the presence of ultramicropores with a radius of less than 0.1 mm. Based on these findings, tests were carried out to change the porous structure of stone as a means of increasing durability. The ultramicropores of stone were filled with neutral matter to lessen the access of water and water solutions of salts and acids while preserving air and vapor permeability. As a result, considerable increase of stone durability takes place, measured by decreased loss of stone weight during the tests. For example, the volcanic tuff of Byurakan type without saturation would have completely destructed after 15 cycles of testing in a saturated solution of sodium sulfate. The same tuff, after its ultramicropores had been filled with a diluted solution of ethyl silicate, have withstood tests with only 4. 4% loss of weight.