On the orders of nanometers, diffusion and adsorption effects deviate significantly from current conventional models, which results in limited estimation accuracy of hydrocarbon capacity and recovery potential. At such small pore sizes, limited pore space as well as wall superimposition effects affect both adsorption and diffusion behavior. Classical laboratory experiments carried out on nanoporous material are unable to characterize the adsorption behavior accurately due to the presence of larger macropores within the network. At present, grand canonical Monte Carlo molecular simulations are usually employed to model both diffusion and adsorption effects independently. However, this method is computationally expensive and does not provide a quantitative variation trend of adsorption isotherms with pore size. In this article, a thermodynamics-based two-dimensional equation of state (2-D EoS) model is introduced to characterize adsorption isotherms in small carbon and mineral capillaries. The effects of pore sizes on adsorption isotherm parameters are compared with those determined from a bulk scale predictive model. Regressed parameters from pure component isotherms are applied to a methane-carbon dioxide binary system. Results are consistent with those determined from molecular simulations. Therefore, it is concluded that by applying the appropriate cubic equation of state, the 2-D EoS adsorption isotherm model is able to produce an accurate quantitative representation of the adsorption behavior in nanopores. Finally, the thermodynamic factor (the correction factor for diffusivity for pure and binary component) is calculated directly from adsorption isotherms. The thermodynamic factor was also shown to have the ability to calculate diffusion coefficients without considering the type of transport mechanism.