This document provides the second edition of Guidance Document 23 on Aqueous-Phase Aquatic Toxicity Testing of Difficult Test Chemicals.

Whilst this guidance is targeted towards tests described by the OECD Test Guidelines identified it may also be relevant to other tests. The guidance relates to the practical aspects of carrying out valid tests with “difficult” test chemicals and mixtures and presenting the results. The guidance is considered to reflect good practice but it is important to recognise that some test chemicals and mixtures will present specific scientific and technical issues that may fall outside its scope. This document is divided into five substantive sections covering guidance for carrying out a preliminary assessment of test chemical stability; general considerations on selection of exposure systems; stock and test solution preparation and exposure systems; sampling of test solution for analysis; and calculating and reporting test results.