Practical applications require hydrothermally stable metal-organic frameworks (MOFs). Achieving stable MOFs in the presence of water or humidity is challenging, especially for MOFs with open metals sites (OMSs) due to the high affinity of water molecules towards OMSs. A straightforward solution to tackling this problem is to protect OMSs in the porous structure of MOFs. A facile post-synthetic modification (PSM) method for the synthesis of molecular glycine-doped CuBTC MOF (BTC = benzene-1,3,5-tricarboxylic acid) was developed in this study. Developed materials, i.e. Gly-CuBTC MOFs, were characterized using various characterization techniques and evaluated using single-component gas (CO2 and N2) adsorption and dynamic water vapor adsorption experiments. The economical dopant of molecular glycine with amine and carboxyl group was found to be able to saturate OMSs in the parent CuBTC MOF leading to the improved hydrothermal stability and CO2:N2 selectivity. It was also found that the adsorption capacity, CO2:N2 selectivity and hydrothermal stability of Gly-CuBTC MOFs depend on the percentage of saturation of OMSs in the parent MOF.