Specific detection of ammonia present at trace level concentrations in a simulated atmosphere was attempted through the combined use of intermodulation CO2laser photoacoustic Stark spectroscopy and phase-sensitive detection at the sum and difference sidebands of the modulating laser and Stark electric fields. Results of experiments performed at both 10R(6) and 10R(8) laser lines indicate the signal dependence on the water vapor concentration present in the used gas mixture. Adhering of ammonia to the cell walls was found also to be a function of the amount of water vapor in the simulated atmosphere.