After two previous investigations demonstrated that the baffled flask test was an effective and reproducible method for screening the effectiveness of dispersant products in the laboratory, the US Environmental Protection Agency decided that before the new protocol could be considered for replacement of the swirling flask test, it would have to be subjected to an interlaboratory round robin investigation. This paper describes how the round robin evaluation was carried out and presents repeatability and reproducibility calculations that demonstrate the superiority of the new method over previous methods. Probabilities for passing various threshold levels of effectiveness values were computed, and the results will be used by the Agency to develop final pass-fail decision rules for dispersant manufacturers wishing to have their products listed on the national contingency plan product schedule. Published by Elsevier Science Ltd.