HOCl is an important intermediate in stratospheric and tropospheric chemistry. Although it can be readily measured in laboratory systems at low pressures (less than or equal to 20 Torr) using conventional electron impact ionization mass spectrometry, there is a need for a measurement technique that can operate at higher pressures, up to 1 atm in air, One such technique seeing increasing use is atmospheric pressure ionization mass spectrometry (API-MS). We report here studies of the API-MS of similar to 0.5-50 ppm HOCl at a total pressure of 1 atm and room temperature. Major peaks from the ion-adducts with Cl- and OCl- were observed. The Br- adduct of HOCl can also be generated using bromoform in the discharge region of the ion source. At the lower range of HOCl concentrations studied in air, the O-2(-) adduct and small parent peaks assigned to HOCl- were observed. The species present as minor impurities in the HOCl source (Cl-2, Cl2O and HCl) can be readily distinguished through identification of the parent ion for Cl-2, or as their adducts with Cl- and Br- for Cl2O and HCl. The identification of HOCl was confirmed using electron impact ionization time-of-flight mass spectrometry (EI-MS). HOCl was quantified using EI-MS to measure the Cl-2 generated when the HOCl reacted heterogeneously on a water-ice/HCl surface and independently by photolysis of the HOCl to generate atomic chlorine, which was trapped using propene and measured as chloroacetone. The implications for the use of API-MS for measuring HOCl in laboratory systems and in ambient air are discussed.