1. The possibility of determining indigo carmine by direct titration with a standard solution of ceric sulphate has been investigated. 2. It has been found that although a rapid direct titration is possible, the amount of ceric sulphate consumed is less than the stoichiometric amount required by the equation representing the oxidation of indigo carmine to isatin sulphonate. However, it has been observed that the amounts of indigo carmine as determined by the ceric sulphate titration are proportionate in themselves, provided all the titrations are carried out at the same dilution. The true values can also be obtained by the use of a correction factor. At a dilution of 0.02 N indigo carmine, the correction factor has an average value of 1.165. This does not vary with the concentration of sulphuric acid at which the titration is made, but shows a slight variation with the concentration of the indigo carmine solution. 3. Reverse titration of ceric sulphate with indigo carmine is also possible, but has been found to be subject to the same limitations.