Positron annihilation lifetime spectroscopy (PALS) is a powerful technique of porosimetry. In this thesis, PALS was used as a complementary technique of porosimetry to overcome the limitations of the conventional tools. Three different porous systems (metal-organic frameworks (MOFs), mesoporous silica, and low-k dielectric layers) have been investigated by PALS as a main tool. PALS helped for understanding the behavior of some MOFs (pore collapse, porosity instability, and the high gas uptake of certain MOFs). On the other hand, investigation of the pseudomorphic transformation process of three different systems by PALS, for the first time, gave very interesting findings and helped for understanding the mechanism of the transformation process. Slow positron beam was used to know the minimum curing time to form porous dielectric layer by the self-assembling approach and it showed that the formed pores are open and their sizes and concentration depend of the curing time and the depth.