Determination of chemical oxygen demand (COD) of samples having high suspended solids concentration such as cattle dung slurry with open reflux method of APHA-AWWA-WPCF did not give consistent results. This study presents a modification of the open reflux method (APHA-AWWA-WPCF) to make it suitable for samples with high percentage of suspended solids. The new method is based on a different technique of sample preparation, modified quantities of reagents and higher reflux time as compared to the existing open reflux method. For samples having solids contents of 14.0 g/l or higher, the modified method was found to give higher value of COD with much higher consistency and accuracy as compared to the existing open reflux method. (c) 2005 Elsevier Ltd. All rights reserved.