The accuracy and precision of electrochemical concentration cell ozonesondes using 1.5% KI sensing solution were measured as a function of pressure altitude from 800 to 6 mbar. Measurements were made in an environmental chamber designed to monitor the instruments' response while subjecting them to ozone, pressure, and temperature profiles typical of those encountered during balloon-borne soundings. Data from dual-instrument balloon soundings were used to provide an independent estimate of precision. The precision was found to be 6--10% (one standard deviation) from 800 to 200 mbar, 5--6% from 200 to 10 mbar, and thereafter increasing to 16% at 6 mbar. The accuracy profile indicated a 3--5% positive error from 300 to 50 mbar. Larger positive errors were observed from 800 to 300 mbar (8--14%) and from 50 to about 15 mbar (10%). The error curve then rapidly rolled off toward negative values, reaching -17% at 6 mbar. Use of new pumping efficiency measurement techniques should improve both accuracy and precision by 2--3% at the lowest pressures but would increase the accuracy error in the 50- to 15-mbar range by about the same amount.