Surface acoustic wave (SAW) devices are very attractive as gas sensors due to high sensitivity and low cost. Polypyrrole, obtained by a solid reaction from a ferric stearate Langmuir-Blodgett (LB) film, has been deposited onto 128 degrees YX-LiNbO3 substrate in order to implement a SAW chemical sensor. The formation of the conducting polymer has been confirmed by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and scanning electron microscopy (SEM) measurements. The SAW delay line, equipped by interdigital transducers operating at a frequency of 42 MHz and coated by a sensitive polypyrrole LB film, has been tested towards NH3 gas in air. Due to the absorption process, the various loadings cause a variation of acoustic wave velocity measured by the phase shift technique at the output of SAW device. Partially reversible SAW acoustic response towards NH3 in air has showed high sensitivity. The calibration curve towards NH3, its detection limit and degradation phenomena of the coating have been performed. Our LB polypyrrole SAW detector exhibits excellent selectivity towards NH3.