The Brewer-Mast ozonesonde was used at Canadian stations from 1966 until 1980, when the Canadian network switched to the electrochemical concentration cell sonde. While the sondes appear to agree relatively well in the stratosphere, there is an evident discrepancy of 10--20% in tropospheric measurements Tarasick et al., 1995, Figure 4>. Comparison of Brewer-Mast sondes with a calibrated ozone source yields some interesting insight into this discrepancy. Sonde response is strongly dependent on the preflight preparation procedures employed. Although sondes prepared via procedures introduced in the 1980s <Claude et al., 1987> perform quite well, when prepared according to the procedures used in Canada in the 1970s, Brewer-Mast sondes indicate 10--30% lower ozone than the calibrator. The following points are noted in particular: (1) a new Brewer-Mast sonde shows a large (~15%) increase in sensitivity between successive experiments; (2) especially at low (<100 ppb) O3 levels, the response even of previously flown sondes increases slowly with time; and (3) sondes show an additional slow increase of response with time that is apparently caused by ozone reactions with the phosphate buffer. The overall response curve indicated by 1, 2, and 3 implies that after correction to the observed total ozone, the earlier part of a flight would yield values that are too low, while the latter part would be too high. By applying a varying ozone input, simulating the typical variation in absolute ozone concentration experienced by a sonde in flight, we show that this can explain both the average correction factor (1.255) for the Canadian Brewer-Mast record and the 10--20% discrepancy in tropospheric measurements.