Abstract Multidentate chelating agents such as NTA, EDTA and DTPA are receiving widespread use in a variety of industrial applications and are entering natural water systems. The presence of these chelates in the environment can be undesirable because they solubilise toxic heavy metals. We have analysed the relative biodegradabilities of NTA, EDTA and DTPA in several different chemical environments. The objective was to determine whether any particular chelate is significantly more biodegradable than the others and therefore more desirable from an environmental point of view. Our results suggest that total degradation (including biological and non-biological) rates decrease in the order: DTPA > EDTA > NTA over the short term and NTA ∼ DTPA > EDTA over the long term. However, photolysis appeared to account for a significant proportion of DTPA degradation. Therefore, considering only biodegradation: NTA > EDTA ∼ DTPA. Degradation rates of all three chelates are not rapid enough, even under ideal laboratory conditions, to preclude concern about their release to the environment.