During the last few years, increases in tropospheric ozone concentration have been detected and the need for more study has been recognized. There is very little knowledge about surface ozone background concentrations prior to the advent of widespread gasoline combustion. Therefore, the aim of this study was to uncover any useful information from the nineteenth century, and to explore the feasibility of converting it to obtain an approximate level of ozone concentration. After discovering ozone, Schönbein promulgated a simple method using iodized starch paper for qualitatively assessing its amount present in the air. This method was implemented at a few hundred sites, and although vulnerable to influence by humidity and oxidants in the air, continued to be used into the early years of this century. In the search for the best method of converting the Schönbein data, the most useful data sources turned out to be the 31-year series of regular quantitative ozone concentration measurements at Montsour... Abstract During the last few years, increases in tropospheric ozone concentration have been detected and the need for more study has been recognized. There is very little knowledge about surface ozone background concentrations prior to the advent of widespread gasoline combustion. Therefore, the aim of this study was to uncover any useful information from the nineteenth century, and to explore the feasibility of converting it to obtain an approximate level of ozone concentration. After discovering ozone, Schönbein promulgated a simple method using iodized starch paper for qualitatively assessing its amount present in the air. This method was implemented at a few hundred sites, and although vulnerable to influence by humidity and oxidants in the air, continued to be used into the early years of this century. In the search for the best method of converting the Schönbein data, the most useful data sources turned out to be the 31-year series of regular quantitative ozone concentration measurements at Montsour...