Six different measurement methods (and seven instruments) for the measurement of gaseous ammonia at low part-per-billion levels were compared simultaneously in a laboratory setting. The instruments were the tunable diode laser (TDL) absorption spectrometer, the wet scrubbing long-path absorption photometer (LOPAP), the wet effusive diffusion denuder (WEDD), the ion mobility spectrometer (IMS), the Nitrolux laser acousto-optical absorption analyzer, and a modified chemiluminescence analyzer. With the exception of the modified chemiluminescence analyzer, the instruments performed well and, understable calibration conditions, generally agreed to within about 25% of the expected calibration value. Instrument time response is shown to be sensitive to measurement history as well as the sample handling materials and is shortest for the TDL. The IMS and Nitrolux are commercial instruments used without modification from the manufacturer. These two instruments have significantly slower time response than the TDL (especially in the case of the Nitrolux) and exhibited measurement biases of approximately +25% (IMS) and -25% (Nitrolux). The LOPAP and WEDD instruments, both research instruments using wet chemical methods, performed well in the calibration tests in terms of the absolute accuracy of measured concentrations, but the WEDD instrument suffered from significantly slower time response than the LOPAP.