Halogenated polymers have emerged as interesting materials for membrane gas separation. Herein, we demonstrate that a combination of F and Cl substituents in polymers provide unexpectedly superior He/gas separation properties, as exemplified by polychlorotrifluoroethylene (PCTFE). PCTFE exhibits a He permeability of 26 Barrers at 35 degrees C and pure gas selectivity of He/H-2, He/CO2, and He/CH4 of 6.2, 53, and 1100, respectively. These selectivity values are among the highest reported for polymers, and the separation performance is very close to the Robeson's upper bounds. The effect of crystallinity in PCTFE on gas transport properties is discussed, and the gas transport properties in PCTFE are compared with other polyethylene (PE) analogues such as PE, polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE). The effect of F and Cl substituents on gas solubility is also elucidated using a variety of hydrocarbons and halogenated liquids. The F substituents lead to unexpectedly high He solubility and exhibit unfavorable interactions with H-2 and CH4, while the Cl substituents increase solubility parameter and size-sieving ability, both of which contribute to the superior He/gas separation properties (particularly He/H-2 and He/CH4). The understanding of the role of F and Cl substituents of polymers in gas transport properties can be useful in designing high performance polymers for membrane gas separation.