Thin films of tin oxide-intercalated polyaniline nanocomposite have been deposited at room temperature, through solution route technique. The as-grown films were studied for some of the useful physicochemical properties, making use of XRD, FTIR, SEM, etc. and optical methods. XRD studies showed peak broadening and the peak positions shift fromstandard values, indicating presence of tin oxide in nanoparticles form in the polyaniline (PANI) matrix. FTIR study shows presence of the Sn–O–Sn vibrational peak and characteristic vibrational peaks of PANI. Study of SEM micrograph revealed that the composite particles have irregular shape and size with micellar templates of PANI around them. AFM images show topographical features of the nanocomposite similar to SEM images but at higher resolution. Optical absorbance studies show shifting of the characteristics peaks for PANI, which may be due to presence of tin oxide in PANI matrix. On exposure to ammonia gas (100–500ppm in air) at room temperature, it was found that the PANI film resistance increases, while that of the nanocomposite (PANI + SnO₂) film decreases from the respective unexposed value. These changes on removal of ammonia gas are reversible in nature, and the composite films showed good sensitivity with relatively faster response/recovery time.