Uccle is a European station with a long tradition in measuring ozone profiles with Brewer-Mast (BM) ozone sensors. In 1996 the use of model Z electrochemical concentration cell (Z-ECC) ozonesondes, manufactured by EN-SCI Corporation, was considered. To document the changeover from BM to Z-ECC ozonesondes, a program of double soundings was started in October 1996. With the use of an appropriate correction procedure, accounting for the loss of pump efficiency with decreasing pressure and temperature, it is possible to reduce the mean difference between ozone profiles obtained with both types of sondes below 3%, which is statistically insignificant over nearly the whole operational altitude range (from the ground to 32 km). It is concluded that with this new method the uncertainty, introduced in trend analyses by the changeover, is less than the internal uncertainty within the BM time series. The required corrections are discussed according to measurements of the pump efficiencies at different pressures and temperatures in a vacuum chamber.