We are presenting applications of a new technique for the study of porosity and diffusivity in rocks employing impregnation with labeled polymethylmethacrylate. We studied samples of different rock types and rock that has undergone various types of alteration. We have focussed special attention on fissures and fissure fillings. Association of porosity with certain mineral phases has been addressed. The spatial distribution of rock porosity exhibits a great diversity. It is a rock-typical parameter. Altered rock is, usually, characterized by inhomogenities in porosity and anisotropy of migration pathways.