Single-walled carbon nanotube (SWNT) has been highlighted as a promising gas sensing element due to its unique properties such as one dimensional electronic structure and high surface to volume ratio. Even though the sensor responses from a number of SWNT gas sensors are at least partially irreversible, little attention has been paid to the nature of the irreversible sensor response. In this work, we demonstrate a shift from an irreversible sensor response to a reversible one via the chemical treatment of a dielectrophoretically formed SWNT network. Both Raman spectroscopy and atomic force microscopy (AFM) show that this shift is related to the nanotube aggregation as well as the percolation behavior. A real-time monitoring of Raman features of metallic and semiconducting SWNT further explains the sensing mechanism.