Herein, we develop a toy mathematical and probabilistic model of a catalytic filter in series with a gas sensor. See Fig. . We abstract (i) the catalytic filter as an isothermal, tubular reactor packed with catalyst particles promoting decomposition of the interferent and analyte with firstorder kinetics and reactant-selectivity for the interferent and (ii) the cross-sensitive gas sensor as gravimetric , whose response is the mass of gas, comprised of both analyte and interferent, adsorbed on its recognition element, governed by the competitive Langmuir adsorption model and corrupted by noise.