A challenge in the widespread adoption of organic solvent reverse osmosis (OSRO) membrane technologies in the chemical and petrochemical industries is the inability to accurately model membranes in contact with complex feeds. Moreover, there is often debate regarding how these membranes separate various organic compounds. We suggest that OSRO membranes typically operate in a solution-diffusion modality, where the permeation rates of various compounds can be estimated based on combinations of pure component diffusion coefficients and their corresponding sorption isotherms in the membrane material. Expectations based on knowledge of organic solvent nanofiltration (OSN) suggest that OSRO membranes may separate compounds predominantly via size-based or diffusion-based mechanisms. In this talk, we will show that a “cohort diffusion” mechanism in which all permeating compounds exhibit the same average diffusivity can accurately predict effective a variety of OSRO separations. The existence of a cohort diffusion modality necessarily implies that the separation occurs exclusively via a sorption-selective modality. We highlight several equilibrium-based methods to estimate sorption selectivity, and these methods are easily amenable to thermodynamic modeling approaches, thus significantly simplifying membrane modeling. Overall, the talk will focus on comparing OSRO membrane measurements with solution-diffusion membrane models with the goal of creating a straightforward model that can be implemented into process simulators and used by process engineers.