As a rule, aquatic organisms are exposed against a multitude of chemicals, either simultaneously or in sequence, rather than against single substances. To predict and assess the toxicity of chemical mixtures based on the single substance toxicity, two concepts, concentration addition and independent action, are mainly discussed. The present dissertation deals with the predictability of the toxicity of a chemical mixture likely to be found in the environment, i. e. constituted of substances with mainly unknown modes of action. The EEC priority List 1 (Council Directive 76/464/EEC) build the basis for the selection of mixture components. To determine the toxicity of the single substances and the mixtures a chronic one generation algae biotest with Scenedesmus vacuolatus as test organism was used. The influence of varying hydrogen concentrations on the predictability and assessment of the combined effect of the mixture of concern was studied. Furthermore experiments were conducted intending to better understand the background of pH-depending toxicity of ionogenic substances. To clarify the impact of low effect concentrations on the toxicity of chemical mixtures, it was examined, whether a mixture, consisting of components at concentrations, which are equal or below their statistically determined effect thresholds, still generates a significant toxicity. The observed toxicty of the mixture composed of 11 chemicals with heterogeneous structures and primarily unknown modes of action was more precisely predictable with the concept of independent action than with the concept of concentration addition. In future, for the prediction and assessment of combined effects of mixtures of environmental chemicals the concept of independent action should be taken into account. Are the mixture components present in concentrations below or equal the individual NOEC of the single substances, a toxicity was observed, which lies significantly above the effects of the single chemicals. Thus it was demonstrated, that thresholds in form of NOECs do not prevent from unwanted mixture effects. The pH-value of the medium, mediated by the pH-depending toxicity of TCP, shows a significant impact on the intensity, the predictability and the assessment of the combined effect of the chemical mixture. Considering this, the validity of the predictability and assessment of the combined effect of the mixture under concern can be crucially improved. The results of the study on the background of the pH-depending toxicity of ionogenic substances could not be interpreted uniformly for all examined substances neither unequivocally in the sense of a sole effectivity of the undissociated form nor in the sense of a contribution of the effectivity of the dissociated form at the overall effect nor in the sense of a concentration additive contribution of both species.