We report an experimental investigation of the adsorption properties of two important small-pore metal–organic framework (MOF) materials recently identified for gas separation applications, through the development and use of a high-pressure/high-temperature quartz crystal microbalance (QCM) device. In particular, we characterize in detail the CO2, CH4, and N2 adsorption characteristics of the MOFs Cu(4,4′-(hexafluoroisopropylidene)bisbenzoate)1.5 (referred to as Cu–hfipbb) and zeolitic imidazolate framework-90 (ZIF-90). We first describe the construction of a QCM-based adsorption measurement apparatus. Single-component adsorption isotherms of CO2, CH4, and N2 in the two MOFs were then measured at temperatures ranging from 30 to 70 °C and pressures ranging from 0.3 to 110 psi. In both materials, the order of adsorption strength is CO2 > CH4 > N2. We find that adsorption in the 1-D channels of Cu–hfipbb can be well described by a single-site Langmuir model. On the other hand, adsorption in ZIF-90 follows a ...