Abstract—Quantum yields for luminol (3‐aminophthalic hydrazide) chemiluminescence reactions fall into two classes depending on oxidizing conditions. In aprotic solvents the quantum yield is high and the excitation yield which allows for the fluorescence quantum yield of the product, is 0·09 and is unaffected by changes in solution temperature or polarity, or the presence of quenchers. In aqueous solution under optimum pH conditions (11–13), hydrogen peroxide oxidation results in a high chemiluminescence quantum yield with an excitation yield of 0·04 again unaffected by temperature, viscosity or quenchers. Other oxidizing conditions produce lower quantum yields probably by the introduction of competing chemical pathways.The luminol chemiluminescence light standard has been used to calibrate a spectrofluoro‐meter with results in good agreement with the quantum yields of the ferrioxalate actinometer and the fluorescence of quinine sulfate and diphenyla