Ozone profiles obtained from the Central Equatorial Pacific Experiment (CEPEX) campaign in 1993 show near-zero ozone levels in the upper tropical troposphere. Ozone chemical destruction and undiluted vertical transport into the upper troposphere were thought to be the cause for the low ozone values found. Questions surfaced as to whether the correct value for the background current measurement was used in the ozone concentration calculations. In this study, three different sensing solutions, including the solution used during that campaign, were tested in the electrochemical cell of the ozonesonde in order to determine the background current generated by the cell as a function of ozone concentration. Two sets of laboratory experiments were performed: In the first set, ozonesondes were tested with purified air, and in the second set, ozonesondes were exposed to a defined amount of ozone using an ozone calibrator. Different ozone amounts were used to determine the background current as a function of ozone concentration. Linear regression of the background current measurements was used to determine a new background correction for the CEPEX raw data. These reprocessed data no longer show near-zero ozone levels in the upper troposphere and no longer require significant chemical ozone loss in the upper troposphere or undiluted transport from the surface for their explanation.