sensor response m op despite x = x op and thus are undetectable. We showed how to determine the local unresponsive subspace of gas composition space from the null space of the Jacobian matrix J of the adsorption model a(x). Whereupon small changes in gas composition in an unresponsive direction, the sensor response remains constant. More, the right singular vectors of J associated with its non-zero singular values give directions in gas compositions to which the sensor is responsive, ranked by sensitivity.