The earlier experiments with potassium ferrioxalate have been extended and a detailed study has been made of the photolysis of the acidified solutions previously recommended for chemical actinometry (Parker 1953). Accurate values of quantum efficiency have been determined at twelve wavelengths between 254 and 578 m$\mu $, and the effect of temperature, light intensity and of photolyte composition have been investigated. At some wavelengths independent values of quantum efficiency have been obtained by comparison with uranyl oxalate and by direct comparison with a calibrated thermopile. The results indicate that potassium ferrioxalate provides a very valuable new chemical actinometer, and its use is recommended in place of uranyl oxalate for many purposes. Full working details for the use of the actinometer are provided.