The standard method for measuring chemical oxygen demand (COD), which is widely used throughout environmental engineering is affected by a number of inorganic substances. These are outlined in Standard Methods (APHA, Standard Methods for the Examination of Water and Wastewater, 16th edition. Washington, D.C., 1985) and methods of overcoming the problems are given, however, no reference is made to hydrogen peroxide as an interfering substance. In a number of complex industrial wastewaters, H2O2 is present and is also used in its treatment which will thus interfere with the analysis. This interference has a positive error effected on COD. This work has been carried out in order to show how hydrogen peroxide interferes in COD analysis, and further shows how to allow for this interference in future COD analysis. A number of specific points have been raised. It has been shown that H2O2 forms a complex with potassium dichromate, the structure of which is given. In experiments which were made on a range of synthetic samples, it was noted that the samples containing H2O2 had different COD values for various concentrations although they contained no organic substances. Also, it was shown on industrial wastewater samples that H2O2 either increased the COD values of the wastewater samples or interfered with the procedure, by completely masking the titration end-point. Reference is made to those industries and processes which contribute to interference by H2O2. A method of calculating the effect of interference by H2O2 is recommended.