An ammonia monitor designed for in situ smoke stack or exhaust duct applications is discussed here. A probe composed of a diffusion cell with a protected multipass optical measurement cavity provides the optical interaction with the sample. Other components of the system include signal processing electronics and an embedded PC104 computer platform. This instrument is useful in a wide variety of ammonia monitoring and process control applications, particularly ammonia-based NOx control technologies, such as selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR). The in situ design eliminates sample handling problems, associated with extractive analysis of ammonia, such as sample line adsorption and heated sample trains and cells. The sensor technology exploited in this instrument is second harmonic spectroscopy using a near infrared diode laser. Data collected during field trials involving both SCR and SNCR applications demonstrate the feasibility and robust operation of this instrument in traditionally problematic operating environments. The instrument can measure other gases by changing the wavelength, either by changing the diode operational set point or by changing the diode. In addition, with straightforward modification the instrument can measure multiple species.