An optimized acid hydrolysis method was developed to yield carboxylic acid-functionalized PIM-1 (PIM-COOH) with >89% conversion in 48 h using a postpolymerization reaction of PIM-1. Physical characterization of PIM-1 and PIM-COOH revealed that the average size of free volume elements in PIM-COOH decreased relative to that in PIM-1. Compared to PIM-1, PIM-COOH showed a significant increase in CO2- and H-2-based selectivities with a corresponding decrease in permeabilities and sorption capacities for all gases considered. The dual-mode sorption model, time-lag method, and sorption-diffusion model were applied to glean molecular-level insights into diffusion and sorption in these polymers. Results indicate that improvements in selectivities for CO2-based gas pairs for PIM-COOH are primarily driven by diffusion selectivity and that PIM-COOH displays transport behavior consistent with the sorption-diffusion model. To better understand performance under more realistic conditions, pure- and mixed-gas permeation values for CO2/CH4 are reported for a 330 day aged PIM-COOH sample.