A comparative study of surface ozone observed at Ahmedabad during 1954--55 and 1991--93 periods has been made. The 1954--55 measurements were made at 0600, 1000, 1400, 1800, and 2100 hours (IST) by an electro-chemical method, whereas 1991--93 measurements are made at every 15 minutes interval by an UV absorption technique. Higher ozone concentrations including larger amplitudes of diurnal and seasonal variations have been observed during 1991--93 as compared to the measurements made during 1954--55. The observed seasonal maximum during 1954--55 is in spring months whereas during 1991--93, it is in winter months. Increase in the concentration of ozone from 0600 hrs in the morning to 1400 hrs in the noon time is higher in summer and October--November months for both the periods and lowest during monsoon season. Furthermore, this change is higher during 1991--93 period as compared to 1954--55 suggesting higher levels of ozone precursors. The average ozone concentration increased from 14.7 ppbv during 1954--55 to 25.3 ppbv during 1991--93, resulting in a linear increase of 1.45% per year. The increase in background ozone concentrations, for 2100 and 0600 hrs during monsoon months when there is low photochemical ozone production even during day time, is 0.49% per year. ¿ American Geophysical Union 1996