In this paper, 10-years of ozone (O3) hourly concentrations collected over the period 2000e2009 in theIberian Peninsula (IP) are analyzed using records from 11 background sites. All the selected monitoringstations present an acquisition efficiency above 85%. The changes in surface ozone over the IberianPeninsula are examined by means of quantile regression, which allows to analyse the trends not only inthe mean but in the overall data distribution. In addition, the ozone hourly concentrations records areclustered on the basis of their resulting distributions.The analysis showed that high altitude stations (>900 m) have higher background O3 concentrations(w80 mg m 3). The same magnitude of background O3 concentrations is found in stations near theMediterranean Sea. On the other hand, the rural stations near the Atlantic coast present lower backgroundvalues (w50e60 mg m 3) than those of Mediterranean influence. The two sub-urban stationsexhibit the lowest background concentrations (w45 mg m 3). The results of the quantile regression showa very distinct behaviour of the data distribution, the slopes for a fixed quantile are not the same over IP,reflecting the spatial dependence of O3 trends. Hence the rate of temporal change is not the same for allparts of the data distribution, as implicitly assumed in ordinary regression. The lower quantile (percentile5) presents higher rates of change than the middle (percentile 50) and the upper quantile (percentile 95).The clustering procedure reveals what has been already detected in the quantile regression. The stationwith highest rates of decrease on the O3 concentrations (easternmost station of IP) is isolated and thenother clusters are formed among the moderately positive/negative O3 trends around the IP. The clusteringprocedure highlighted that the largest trends are found for the lower ozone O3 values, with largestnegative trend at the easternmost station of IP, and also in northern and mainland stations, and anopposite behaviour, with positive O3 trends, is observed at the Atlantic coast stations