Ground level ozone (GLO3) was measured at the Alert, Nunavut research station (82º28’N, 62º30’W) from 31 December 1991 to 31 December 2003. These measurements were conducted to: 1. determine a long term trend in background ozone, and 2. to understand the annual appearance of surface level ozone depletions that start in mid-March ( just after the time of polar sunrise) and extend into June. Ozone depletion events have also been correlated with atmospheric mercury depletion events. Many field studies have been carried out at Alert to unravel the reason for this phenomenon. As a result, accurate measurements of ozone have been of crucial importance. GLO3 was measured with a commercial Thermo Environmental Instruments Model 49 instrument based on the principle of ultraviolet absorption by ozone. Calibrations were performed approximately every 2 months and were referenced to the National Institute of Standards and Technology, (NIST) in Gaithersburg, Maryland, U.S.A.. Calibrations were usually within 1% of the mean calibration factor and it is estimated that the final hourly corrected O3 mixing ratios are accurate to within ± 2% or ± 1 ppb, whichever is greater. After 2003, Alert GLO3 data are available from the Canadian Air and Precipitation Monitoring Network (CAPMoN).