These laboratory studies were aimed at distinguishing the relative importance of released or added ferric ions and of ferric oxide particles for the photodegradation of oxalic acid in natural waters. The parameters whose effects were determined for this purpose included the pH, the irradiation wavelength, the concentrations of the various species and the identity of the ferric oxide sample. In addition, the existence of a potential photocatalytic activity of ferric oxide was established by measurements of the electrical photoconductance of three samples under vacuum and in a dry dioxygen atmosphere. The results show the dominant role of ferric ions. However, the influence of ferric oxide is clearly not attributable to the mere dissolution. Effects due to the photoexcitation of surface iron-HC 2 O 4 - complexes are confirmed. Photocatalytic events may exist. The relative importance of these various phenomena depends on the[Fe 3 + ] d i s s 2 O 3 ] ratio and on the identity of the ferric oxide sample; in particular the crystallinity seems to favour the photodegradation of oxalic acid.