In this study, we prepared samples of a well-controlled, internally smooth, synthetic material (i.e., porous silica glass) and submitted them to chemical alteration while measuring changes in porosity and permeability. In the early stages of alteration we observed a large drop in permeability, whereas porosity remained essentially unchanged. In the late stages the permeability decrease became gradual so that even small decreases in permeability were accompanied by substantial decreases in porosity, as has been observed previously in natural sandstones and shales. A detailed microstructure study showed that smooth pore walls were roughened during alteration. Furthermore, the pore space heterogeneity increased and the connectivity decreased. A simple conceptual model suggests that the alterations occur while fluids are flowing.