This International Standard provides procedures, not covered by the methods described in ISO 8692 and ISO 10253, for testing difficult substances for inhibition of algal growth. The main subjects covered by the guideline are the methods for preparing the test substance for testing and the procedures needed to carry out an appropriate test. The following test substances are covered by this guideline: a) poorly soluble pure organic compounds b) poorly soluble mixtures of organic substances c) poorly soluble inorganic materials d) volatile substances e) waste waters and environmental samples containing water and sediments f) coloured and/ or turbid samples g) compounds of heavy metals. The following methods of addition are covered: ⎯ direct ⎯ dispersion ⎯ water-soluble and water-accommodated fractions. Some guidelines related to the analytical procedures and to the interpretation of the results have been included. References to documents describing the background for the testing of difficult substances are given in the Bibliography.