We now demonstrate the identification of the unresponsive subspace and ranked responsive directions in gas composition space for a gravimetric, single-MOF sensor and two-MOF sensor array for diluents and higherhydrocarbons in natural gas . Specifically, suppose the sensor (array) is immersed in a variable-composition gas mixture involving N 2 , C 2 H 6 , CO 2 , and CH 4 at 1 bar and 298 K. We consider two candidate MOFs as the recongition element(s), HKUST-1 and Co-MOF-74 (see Fig. ).