Chemical substances can be transformed under the influence of physical/chemical and/or biochemical active factors. By this, the ecotoxicological and hygienic‐toxicological properties and effects of the substances are changed also with respect to toxicity, late effects (carcinogenetic, mutagenic, teratogenetic ones) as well as the bio‐ and geo‐accumulation behaviour. Such transformations occur also during water treatment, and therefore also the properties and effects of the transformation products have to be taken into account for determining the limit values for pollutants in raw water for the production of drinking water. Since frequently we have incompletely known transformation products or mixtures of substances, a coefficient of hazard is suggested for the evaluation. It is the quotient of the threshold concentrations of the initial substance and of the transformation products for the different (organoleptic, toxic, carcinogenic, teratogenetic etc.) effects. For several organic substances it is demonstrated that the transformation products can be regarded less toxic compared with the initial substance. Strong oxidants, however, cause an increased danger in many cases.