Humic substances in aqueous solution undergo characteristic alterations when irradiated with light of wavelengths > 290 nm. Humic substances subjected to a preliminary irradiation are less capable of generating reactive oxygen species. This effect is illustrated by changes in spectroscopic (u.v., ESR) and other physical properties of the samples. | Humic substances in aqueous solution undergo characteristic alterations when irradiated with light of wavelengths > 290 nm. Humic substances subjected to a preliminary irradiation are less capable of generating reactive oxygen species. This effect is illustrated by changes in spectroscopic (u.v., ESR) and other physical properties of the samples.