Chemical oxygen demand (COD) is a critical analytical parameter in waste and wastewater treatment, more specifically in anaerobic digestion, although little is known about the quality of measuring COD of anaerobic digestion samples. Proficiency testing (PT) is a powerful tool that can be used to test the performance achievable in the participants' laboratories, so we carried out a second PT of COD determination in samples considered "difficult" to analyze (i.e. solid samples and liquid samples with high concentrations of suspended solids). The results obtained (based on acceptable z-score values) may be considered satisfactory. When compared with the results of a previous similar scheme, the overall performance improved by around 30%, again demonstrating that analytical performance can be improved by regular participation in PT. (C) 2010 Elsevier Ltd. All rights reserved.