During the period of 1901–1940 regular measurements were made of the surface ozone concentration (SOC) at Athens, using De James photosensitive paper, which provided a unique set of observational data for SOC in South–Eastern Europe. Recent studies have shown, however, that the color sensitivity of the colorimetric paper is greatly dependent on the air humidity, exposure time, and concentration of minor atmospheric gaseous constituents which exhibit oxidation and recovery properties. Because of this a correction procedure has been developed and applied to the early observational data. The resulting set of adjusted data has been subjected to statistical processing in an effort to analyze time variations of SOC. A most important result of the analysis is the fact that SOC was found to increase from 20 ppb in 1901 to approximately 28 ppb in 1920 and then to decrease down to 15 ppb in 1940. In addition, distinct 12–month, 6–month, and 4–month periodicities have been noted in SOC behavior. When the annual cycle of SOC observed at Athens from 1901 to 1940 and 1987–1990 was compared to similar results obtained at other locations, it became apparent that SOC was higher at Hohenpeissenberg (Germany) from 1971 to 1988 than that at Athens throughout the observation period under review. Conversely, the annual cycle at Arosa (Switzerland) as evidenced by the data for 1954–1958 differs but slightly from that observed at Athens from 1901 to 1940. The phases of the annual cycle of SOC at Montsouris (France) from 1977 to 1910 coincide with those observed at Athens whereas the surface ozone values are, on the average, 4 ppb less. The analysis of the surface ozone variation with wind conditions shows that O3 concentration is predominantly determined by the variation in the photochemical ozone production.