Carbon nanotubes (CNTs) have drawn a lot of interest as the sensing element in sensor technology because of their unique electronic properties and remarkable mechanical properties. CNTs' extremely high surface-to-volume ratio makes it a very good candidate for the adsorption of gas molecules. Gas sensors are divided into two types, namely; physical gas sensors and chemical gas sensors. In the case of chemical gas sensors, it is difficult to detect some gases which have low chemical adsorption energy like inert gases since these sensors operate by variation in the sensing element's resistance. Physical gas sensors which operate by ionization mechanism 1, 2, work by fingerprinting the ionization characteristics of distinct gases. Although these ionization sensors have better selectivity and response time, but the ones with traditional electrodes are still huge and bulky. With CNTs as the sensing elements providing billions of sharp nanotips, breakdown voltage can be lowered such that safe operated sensor with small size, good selectivity and sensitivity can be realized. CNT-based gas sensors can also be operated in room temperature which will result in safer environment. The paper reports the fabrication and successful testing of ionization gas sensor using aligned multiwalled carbon nanotubes (MWCNTs) array 3 featuring the electrical breakdown for several gas species namely air, argon and 2% of hydrogen in air. The effect of the electrode separation on the electrical breakdown was also investigated. ©2010 IEEE.