The surface ozone time series is analyzed for the seasonal and inter-annual variations and the trend in the following three categories (a) monthly mean (b) 8 hr monthly mean and (c) daily maximum monthly mean measured at 8 (urban/suburban) sites in Portugal for the period 2000–2010. The inter-annual variation of the monthly mean surface ozone time series showed an year to year variation with the highest value in May 2009 (95 µg m–3) at Monte Chãos and the lowest in Jan 2002 (17 µg m–3) at Paio Pires. The trend analysis of (1) original surface ozone time series and of (2) deseasonalized surface ozone time series for all the three data set categories and for all the 8 sites was performed and found to be statistically significant at 6 sites for monthly mean and 8 hr monthly mean and at 5 sites for daily maximum monthly meanusing the non-parametric Mann-Kendall test. The analysis of original surface ozone time series showed a positive trend at 6 out of 8 sites, but the results were not statistically significant for most of the sites due to the presence of the annual cycle masking the actual trend values. However, the analysis of deseasonalized surface ozone time series showed a statistically significant increasing trend at 7 out of 8 sites with high Z values. The positive trends found in the deseasonalized surface ozone time series were in the range 0.44 up to 1.42 µg m–3 year–1 for the monthly mean surface ozone time series (7 stations), 0.33 up to 1.43 µg m–3 year–1 in the case of the 8 hr monthly mean surface ozone time series (7 stations) and 0.66 up to 1.55 µg m–3 year–1 in the daily maximum monthly mean surface ozone time series (6 stations).