Membrane performances were evaluated in a custom-built Wicke Kallenbach gas permeation setup. The prepared MIL-116 (Ga)-formate PSU membranes were placed in a stainless-steel membrane module (offering an effective membrane area of 4.01 cm 2 ) and sealed gastight by Viton® O-rings to prevent leakage. For practical testing for He separation (4:96 He:CH 4 mixture), He and CH 4 were supplied at flow rates of 4 mL/min and 96 mL/min respectively at 60 °C keeping pressure at 1 bar. The permeate side was swept with N 2 at 1 ml/min. The composition of the permeate side stream was analysed by a gas chromatography system from Shimadzu (GC-2030 Nexis).