A new experimental protocol and molecular level theoretical models for assessing nanoporosity and soil retention ability by using CO2 as a molecular probe was developed and verified. A typical hysteric sorption-desorption isotherm of benzene in aqueous suspensions of Pahokee soil (93% organic matter) was presented. The results showed pronounced hysteresis for benzene in this soil. Sorption hysteresis of organics and sorption hysteresis of CO2 were caused by similar mechanisms and, therefore, CO2 could be used as a suitable molecular probe to study structural and sorption-desorption properties of soil particles containing soil organic matter (SOM). The CO2 adsorption-desorption isotherms showed hysteresis that presumably indicates irreversibility of sorption in SOM nanopores. Hysteresis was especially strong in samples having high organic carbon content: peat and its purified humin and humic acid fractions. The adsorption isotherm on peat and humic acid revealed a bend not observed in humin.