This paper reports the synthesis and characterization of a novel family of copolyimides having adamantyl ester pendant groups, along with their characterization and a study of properties such as solubility, thermal stability, and gas separation capacity. The incorporation of the pendant groups was done through esterification reactions using a copolyimide with free −COOH pendant groups and three adamantane−alcohols with different substitutions. The gas permeation (P) and diffusion (D) coefficients of the three copolyimides were higher than the starting copolyimide and they were governed by the fractional free volume, which confirmed the positive effect of adamantane groups on reducing the molecular packing and therefore increasing P and D. To confirm the above statement, theoretical fractional free volume, density and free volume distribution were calculated observing similar trends to those obtained experimentally. Finally, evaluation of P(CO₂) behavior as a function of CO₂ feed pressure revealed the absence of plasticization, what was attributed to the film densification caused by the thermal annealing.