A general mixed mechanism model formulated within a grid framework is proposed. In particular, the framework is employed to account for the variability in the diffusion of carbon dioxide and methane in a certain class of inorganic-organic membranes. This variability is a basis for separation of gas mixtures into the various components. Various sizes of the cells within the grid are developed for a series of hybrid inorganic-organic membranes. Parameters in each model are estimated from the permeance results of both carbon dioxide and methane. Acceptable grid models are determined by comparing the estimated parameters of a surface diffusion mechanism with the values reported in the open literature. For each membrane, the model with the minimum number of grid cells is studied. The diffusion in the studied membranes is dominated by activated Knudsen at lower temperatures and limited by surface diffusion when the temperature is above a critical value. Activated Knudsen diffusion is the major mechanism that affects carbon dioxide/methane selectivity.