The ozonesonde stations at Uccle (Belgium) and De Bilt (Netherlands), separated by onldy 175 km, offer a unique opportunity to test the influence of different ozonesonde types and different correction strategies, as well as to detect the presence of inhomogeneities in the ozonesonde time series resulting from changes in sounding equipment (solution, radiosonde, ozonesonde, interface, sounding software, etc.). In particular, we highlight a 2.5 year period (beginning of 2007 to mid-2009) of anomalous high tropospheric ozone values measured by ozonesondes at Uccle and compare these with the observations from De Bilt. Because the ozone deviations are onldy observed in the free troposphere where ozone concentrations are relatively low, and not in the boundary layer or the stratosphere, this issue is directly related to the sensitivity of ozonesondes. Therefore, the effect of every instrumental change, even though small, during this 2.5 year anomalous period is analyzed considering a change in the radiosounding equipment, different ozonesonde batches, operational differences at the stations, differences on ascent and descent during the anomalous period; an environmental cause is also examined. Unfortunately, one single, specific cause for the observed high tropospheric ozone values at Uccle could not be identified. There are two explanations consistent with the observations and not ruled out by the analysis here: 1) the majority of the ozonesondes used at Uccle between March 2007 and August 2009 needed longer conditioning of their sensors and, therefore, behaved more accurately at low ozone concentrations during their descent or when used a second time, and 2) an environmental origin arising from a local difference in the air mass between Uccle and De Bilt and between the ascent and descent.