Limited information is available on microporosity in soils. A study was undertaken to investigate the micropore characteristics of four soil samples, with different particle-size distributions, and four natural silicate clay minerals. Specific surface area and the differential micropore-size distribution were taken into account to characterize the microstructure of the soils and clays. The micropore-size distributions showed a maximum contribution to the total microporosity by pores having an effective pore radius of ~20 Å, thus indicating that that category of pore contributes more than others to the total microporosity of the system. For both soils and clays a good exponential correlation was found between the maximum contribution to the microporosity and their specific surface area. A linear relationship was also found between the microporosity of the soils and their clay content. It has been concluded that the micropore system formed by 20 Å pores is mainly located in the clay fraction of the soil, and contributes significantly to defining some of the most notable physicochemical properties of soils and clays.