The pore structure of two types of tuff was measured with a mercury porosimeter. The high percentage of adsorption pores explains the disruptive effect of wetting-drying previously observed; differences in pore-size distribution shown by two types of tuff also in correlates with their different resistance to this stress. Salt crystallization pressure was calculated on the basis of pore-size distribution; the results are in agreement with the results of a previous laboratory study and observations in the field regarding one of the tuffs in particular. The results show the clear relationship between texture and mechanical failure of these stones. The possibility of and requirements for preservation treatments are also discussed.