Selective molecular sieving through porous graphene membranes is demonstrated by introducing subnanometer-sized pores via UV-induced oxidative etching. The resulting membranes act as molecular sieves, selectively allowing smaller gas molecules to pass while blocking larger ones. The membranes are fabricated by mechanical exfoliation of graphene over predefined silicon oxide wells, forming suspended membranes that are impermeable to standard gases. After pressurization with a desired gas, the membranes are allowed to equilibrate, and the resulting deflection is measured to determine gas leak rates. UV etching introduces pores that enable selective permeation, with the leak rates and separation factors agreeing with models based on effusion through small pores. The membranes are tested using both a pressurized blister test and mechanical resonance, revealing that the etched pores significantly increase the leak rate of small molecules like H₂ and CO₂ while leaving larger molecules like Ar and CH₄ largely unaffected. The molecular selectivity is further confirmed by measuring the time rate of change of deflection for different gases. The results are consistent with theoretical models, showing that the pores enable size-selective transport. The membranes are also compared to computational models, with the measured H₂ leak rate being several orders of magnitude lower than some simulations, suggesting higher energy barriers in the pores. The study highlights the potential of porous graphene membranes for gas separation and represents an important step toward macroscopic, size-selective membranes. The approach can also be used to probe the fundamental limits of gas transport through angstrom-sized pores.Selective molecular sieving through porous graphene membranes is demonstrated by introducing subnanometer-sized pores via UV-induced oxidative etching. The resulting membranes act as molecular sieves, selectively allowing smaller gas molecules to pass while blocking larger ones. The membranes are fabricated by mechanical exfoliation of graphene over predefined silicon oxide wells, forming suspended membranes that are impermeable to standard gases. After pressurization with a desired gas, the membranes are allowed to equilibrate, and the resulting deflection is measured to determine gas leak rates. UV etching introduces pores that enable selective permeation, with the leak rates and separation factors agreeing with models based on effusion through small pores. The membranes are tested using both a pressurized blister test and mechanical resonance, revealing that the etched pores significantly increase the leak rate of small molecules like H₂ and CO₂ while leaving larger molecules like Ar and CH₄ largely unaffected. The molecular selectivity is further confirmed by measuring the time rate of change of deflection for different gases. The results are consistent with theoretical models, showing that the pores enable size-selective transport. The membranes are also compared to computational models, with the measured H₂ leak rate being several orders of magnitude lower than some simulations, suggesting higher energy barriers in the pores. The study highlights the potential of porous graphene membranes for gas separation and represents an important step toward macroscopic, size-selective membranes. The approach can also be used to probe the fundamental limits of gas transport through angstrom-sized pores.