The OECD Secretariat received almost simultaneously three proposals for an OECD Test Guideline on Direct Phototransformation in Water, i.e. aqueous photolysis. The purpose of this Guidance Document is to situate these proposals in a broad context consisting of the basic concepts of photochemistry applicable to aqueous photolysis, the existing national guidelines and standards, and the results of a ring test held under the aegis of ECETOC.