In this article, we outline a mathematical method to identify changes in the gas composition to which reversible ‡, quantitative gas sensors /sensor arrays are unresponsive. Gas sensing can be viewed as the inverse problem of finding the gas composition consistent with the sensor response governed by a mixedgas adsorption model . Owing to crosssensitivity, the mapping, under the adsorption model, from gas composition space to sensor array response space could be non-injective (many-to-one). Consequently, some gas composition changes are undetectable. By our defi- ‡ Gas sensors based on physical adsorption in stable MOFs lacking adsorption/desorption hysteresis are reversible.