In the present work, experimental design was used for the fast optimization of three kinds of sample digestion procedures with the final aim of obtaining the COD value of wastewater samples. The digestion methods evaluated were "closed microwave-assisted" (CMWD), "open microwave-assisted" (OMWD) and "ultrasound-assisted" (USD). Classical digestion was used as reference method. The optimum values for the different variables studied in each method were: 90 psi pressure, 475 W power and 4 min irradiation time (CMWD); 150 degrees C temperature and 4 min irradiation time (OMWD); 90% of maximum nominal power (180 W), 0.9 s (s(-1)) cycles and I min irradiation time (USD). In all cases, interference concentration that produces a deviation of 10% in COD values is 13.4, 23.4, 21.1 and 2819mg/L for S2-, Fe2+, NO2- and Cl-, respectively. Under optimum conditions, the proposed digestion methods have been successfully applied, with the exception of pyridine, to several pure organic compounds and COD recoveries for 10 real wastewater samples were ranged between 8 8 and 104% of the values obtained with the classical (open reflux) method used as reference, with R.S.D. lower than 4% in most cases. Thus, the use of ultrasound energy for COD determination seems to be an interesting and promising alternative to conventional open reflux and microwave-assisted digestion methods used for the same purpose since the instrumentation is simpler, cheaper and safer and the digestion step faster than the ones used for the same purpose. (c) 2006 Elsevier B.V. All rights reserved.