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Biotic ligand models have been developed for various metals (e.g. Cu, Ag, Zn) and different aquatic species. These models incorporate the effect of physico-chemical water characteristics (major cations, pH, dissolved organic carbon) on the bioavailability and toxicity of the metal. In this study, the individual effects of calcium, magnesium, potassium, sodium and pH on zinc toxicity to the green alga Pseudokirchneriella subcapitata (formerly and better known as Selenastrum capricornutum and Raphidocelis subcapitata) were investigated. Stability constants for binding to algal cells (K_BL) were derived for those cations affecting zinc toxicity, using the mathematical approach proposed by De Schamphelaere and Janssen [Environ. Sci. Technol. 63, (2002) 48–54]. Potassium proved to be the only cation tested that did not alter zinc toxicity to algae significantly. Log (K_BL) values for Ca, Mg and Na, derived at pH 7.5, were 3 …