Schönbein’s correspondence with Berzelius (1836–1847) and with Faraday (1836–1862) are shown to be excellent accessible sources for his changing views as to the nature of ozone. Two of Schönbein’s qualitative tests for ozone, namely the reaction with neutral potassium iodide solution and that with indigo have current quantitative application. Andrew’s quantitative studies of the ozone reaction with potassium iodide (1856) and his work with Tait on the volumetric gas phase decomposition of ozone (1860) confirmed the allotropic nature of ozone but did not yield the structure which was first proposed by Odling (1861). Modern spectroscopic studies owe their origin to Hartley’s studies of 1881.