A closed-loop experimental setup was developed to investigate the kinetics of CO2 and H2O coadsorption onto a differential bed comprising amine-functionalized nanofibrillated cellulose (NFC) under conditions relevant to direct air capture. The temporal sorbate uptake profiles were determined in the absence of external heat and mass transfer intrusion based on the mass balance in the constant-volume gas phase. The uncertainty of the mass balance calculations caused by the inherent nonuniform gas pressure and temperature within the tubing, valves, and instrumentation was decreased 4-fold with incorporation of a large perfectly mixed gas tank that occupied 94% of the total volume of the closed loop. The preliminary results have revealed the half-times for H2O and CO2 adsorptions of the order of tens of seconds and tens of minutes, respectively. This finding implies that coadsorption on amine-functionalized NFC can be investigated by considering that the H2O loading on the sorbent is at any instance in the equilibrium with the surrounding humidity. Because of the much faster kinetics, the H2O adsorption becomes intruded by mass transfer at a similar to 10 mm sorbate particle size while the CO2 adsorption remains unaffected. The design and validation methodologies presented in this work may serve as reference for the development of batch experimental setups for measuring kinetics of gas-solid reactions.