Commercially available high quality 9B pencil graphite was used for sensing of trace concentrations of nitrogen dioxide (NO₂) and ammonia (NH₃) at sub-ppm levels in air at ambient temperature and pressure. The pencil graphite was characterized using scanning electron microscopy (SEM), Raman spectroscopy and powder X-ray diffraction (XRD) techniques. In comparison to other sensing materials pencil graphite is very well recognized owing to its cheaper price, simplicity of fabrication, abundant over-the-counter obtainability and easy modification. The principal of operation of the chemiresistive gas sensor is based on the variation in the electrical resistance due to the selective interaction between the pencil graphite network and the specific gaseous analyte in a two-pole format. The degree of change in electrical resistance of the pencil graphite network depends on the concentration of the gaseous analyte to which it is exposed. On the basis of this fact qualitative as well as quantitative detection of the gaseous analytes can be achieved. The sensing limits of 100 parts-per-billion (ppb) and 500 ppb with sensor response times of ∼30 s and ∼50 s for NO₂ and NH₃, respectively is recorded using our gas detector, and are found significantly efficient in comparison to the NO₂ and NH₃ detectors available commercially in market.