A polyaniline (Pani) thin film doped with camphorsulfonic acid (CSA) has been deposited using spin coating technique on a glass substrate. Polyaniline is chemically synthesized by an oxidation method at ∼0°C. Sensing experiments are performed on the Pani-CSA thin film with the injection of 50, 100, 150 and 200ppm level NH 3 at room temperature. The change in resistance, based on the modification of doping level, occurs due to redox interaction of the analyte. Response and recovery times are found to be ∼1s and ∼7min, respectively. The hexagonal structure seen in the scanning electron micrograph is helpful in making the sensor response fast.