Chemiresistive threshold sensor films based on the switchable metal−organic framework (MOF) MIL-53(Al) (MIL = Mateŕiaux de l'Institut Lavoisier) and conductive carbon additives were developed, characterized and successfully applied for selective detection of CO 2 in mixtures with methane at high pressure (up to 25 bar). Two transitions of the crystal structure, from the lp (large pore) form to the np (narrow pore) form at CO 2 partial pressure below 0.5 bar and back to the lp form at ca. 6 bar, also known as "breathing", result in two detectable stepwise resistance changes of sensor film, if CO 2 is present in the mixture with methane. The sensor shows a rapid response of the specific resistance (response time ca. 25 s), which directly correlates to the expected structural change of the switchable MOF. Two films containing carbon or carbon nanotubes as conductive component were compared regarding the influence of the additive particles geometry on sensing performance, showing benefits of the isotropic particle shape. Cyclic sensing measurements proved the durability of the composites.