2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo₂CT_x flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub‐µm MXene multilayer for different organic vapors and humidity at 10¹–10⁴ ppm in dry air is studied. Reasonably, the low‐noise resistance signal allows the detection of H₂O down to 10 ppm. Moreover, humidity suppresses the response of Mo₂CT_x to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10⁻²–10⁶ Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo₂CT_x chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo₂C surface interaction with organic analytes and H₂O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.