Bentonite, biotite, illite, kaolin, vermiculite and zeolite were acidified or alkalized with hydrochloric acid or sodium hydroxide of concentrations 0.0, 0.1, 1.0 and 5.0 mol dm−3 at a room temperature during two weeks. The treated minerals were transferred to calcium homoionic forms from which aggregates were formed by hand (if possible) and subjected to a few cycles of wetting–drying to stabilize the structure. Aggregates of bentonite, illite, kaolin and zeolite, and loose beds of biotite and vermiculite (no aggregation) were studied by mercury porosimetry. From mercury intrusion curves pore volumes, size distributions, average radii and surface fractal dimensions were calculated for pores ranged from c.a. 0.01 to 10 μm in radius. In most cases with an increase of the concentration of acid and alkali the pore volumes increased indicating a loosening of the minerals texture. Depending on the mineral and the treatment both broadening of pore dimensions and formation of new fine pores was observed, therefore the average pore radii changed in different directions. No well defined trends in the pore surface fractal dimension were noted under both treatments, however in most cases they were lower for treated than for nontreated minerals.