Many of the compounds in use today have ionizing properties. Investigations have shown that around half of the compounds preregistered for REACH and over 70% of all pharmaceuticals are ionizing organic compounds. These compounds may pose a risk when they are released into the environment. Ionization, however, complicates the environmental risk assessment of these compounds because the uptake processes of the neutral fraction differ from the processes of the ionized fraction. Acids are increasingly neutral at pH levels below the pKa while bases are increasingly neutral at pH levels above the pKa. Because the neutral fraction is more lipophilic than the ionized fraction, ionizing organic compounds are often taken up more efficiently when they are present in the neutral form. Several studies have thus shown that acids are more toxic and more bioconcentrating at lower pH levels while bases are more toxic and bioconcentrating at higher pH levels. In this study the bioconcentration and toxicity of the bivalent weak base chloroquine was tested on Salix viminalis at pH levels of 6, 7, 8 and 9. It was found that both bioconcentration and toxicity are higher at high pH values where the compound is increasingly neutral. A simulation with the cell model showed similar results. However, to draw any conclusions concerning the behavior of acids and bases in general, it is necessary to move away from the one-compound approach, and start looking at a larger dataset. Therefore a dataset was compiled from an extensive literature search, and based on this dataset, the toxicity and bioconcentration of electrolytes was found to be systematically higher at pH levels that favor the neutral species. In surface waters with pH levels between 6 and 9 the bioconcentration and toxicity of acids and bases can thus be expected to fluctuate with pH if pKa values fall within the range of 3-10 for acids and 5-12 for bases. Toxicity tests with Pseudokirchneriella subcapitata for the acid salicylanilide and the bases trimethoprim and ethoxyquin were in accordance with these ranges. Zwitterions and amphoters show pH dependent toxicity and bioconcentration if they contain a pKa value within the ranges given for acids and bases. Three theoretical exceptions were found to the above. The first exception is when the recipient water has pH levels outside the normal range of pH 6-9. In such cases the given pKa range is too narrow and must be adjusted to accommodate the extreme pH level. The second exception is when the ionized fraction is toxic through a specific mode of action - an example was found in the literature for sulfonamides where it is the ionized species that exert the antibiotic effect. This, however, only presents an exception if the bacteria are unable to maintain homeostasis. Bioconcentration and toxicity experiments with Daphnia magna showed that the sulfonamide sulfadiazine behaves as a simple acid with higher toxicity and bioconcentration at low pH levels. A final theoretical exception was identified for cations with delocalized charges: a group of compounds for which ionized and neutral fractions have equal or similar lipophilicity. The neutral and ionized fractions of these compounds are taken up at equal rates, but the added effect of electrical attraction makes it theoretically possible for the cation to be more toxic and more bioaccumulative. The existence of such an exception, however, could not be verified experimentally with the model compound rhodamine 6G. The role of the ionized fraction for uptake and toxicity is often assumed to be negligible. An examination of the contribution of the ionized fraction to the bioconcentration of ionizing organic compounds showed that this fraction cannot safely be overlooked. The work presented in this thesis suggests that the standard test procedures used to test toxicity and bioconcentration are not sufficient to fully illuminate the ecotoxicity of ionizing organic compounds unless the effect of pH is somehow considered. The final work presented in this thesis is a series of schematic selection criteria designed to facilitate rapid and correct decisions about which pH level to choose for bioconcentration and toxicity experiments. These guidelines are based on the findings disclosed in this thesis, and on the discussion thereof. The regular use of these guidelines will help eliminate situations where bioconcentration or toxicity is underestimated. The guidelines are supported by simple and robust test methods for conducting pH specific toxicity tests with the common test organisms Daphnia magna and Pseudokirchneriella subcapitata in the pH range of 6-9.