The extent to which Ca2+, Mg2+, Na+, K+ ions and pH independently influence copper toxicity to barley (Hordeum vulgare) was assessed by measuring root growth in nutrient solutions. Increased Ca2+ activity resulted in a sixfold decrease in EC50Cu2+ values, while a positive relationship between the cation activity and the EC50 was expected. Increased Mg2+ activity resulted in a twofold increase in EC50Cu2+ values. Na+, K+ and H+ activities did not significantly affect Cu2+ toxicity. The obtained results indicated that competition for binding sites between Cu2+ and cations such as Ca2+, Mg2+, Na+, K+ and H+ is not an important factor in determining Cu2+ toxicity to H. vulgare. However, the EC50s could, with one exception, be predicted within a factor three based on the free Cu2+ activity, indicating that the free Cu2+ activity cannot only be used to predict metal toxicity to aquatic, but also to terrestrial organisms.