Many previous studies have examined structure-property relationships of both polymeric and carbon molecular sieve gas separation membranes, which show a widely known tradeoff between permeability and selectivity. This work presents an explanation of a lesser-known trend of a selectivity maximum as a function of the material structure for certain rigid polymers and a variety of carbon molecular sieving materials. Interestingly, this surprising trend of a maximum in selectivity over a range of materials is observed for certain gas pairs and not others over the same range of material structures. First-order modeling efforts, which treat molecular sieving membranes as effective heterogeneous composite materials comprising a distribution of selective entities, are used to explain and understand this trend of a maximum in selectivity. Several composite models are considered, and specific examples are demonstrated using both a parallel model and effective medium theory. The modeling results reflect trends in experimental data and suggest that subtle changes in the distribution of selective entities or the size of gas molecules can result in vastly different overall separation properties.