Oxygen was used as a source gas in a conventional Innsbruck PTR-MS instrument to produce O2+ ions as chemical ionization (CI) reagents instead of H3O+ ions. The use of O2+ ions as CI reagents allows for fast, highly sensitive and specific measurements of gas-phase ammonia (NH3) via the electron transfer reaction O2+ + NH3 → NH3+ + O2. The instrument was tested to be linear in the 2–2000 ppbv range. Instrument sensitivity was observed to be humidity-independent and amounted to ∼40 cps/ppbv. The instrumental background was determined by sampling NH3-free air from a heated platinum/palladium catalyst. A humidity-dependent increase of the instrumental background from 70 pptv at dry conditions to 470 pptv at humid conditions was observed. The corresponding 2σ-detection limits at 1 s signal integration time were 90 pptv for dry conditions and 230 pptv for humid conditions, respectively. The observed background may be intrinsically formed in the instrument's ion source but it may also be the result of incomplete NH3 oxidation in the catalyst used for zeroing. The reported background levels and detection limits are thus to be considered as upper limits. The 1/e response time of the instrument was in the range of 3–5 s. The PTR-MS instrument was successfully deployed in the field to monitor changes in gas-phase NH3 concentrations in the few seconds to tens of seconds time range. Laboratory intercomparison measurements between the PTR-MS instrument and a commercial NH3 analyzer (AiRRmonia) were in good agreement. The use of O2+ ions as CI reagents will significantly improve the analytical capabilities of the Innsbruck PTR-MS instrument.