Natural waters, in terms of thermodynamics, are an open system in which various redox reactions involving oxidants (molecular oxygen, hydrogen peroxide, free radicals, etc.), reducing agents and transition metal ions are constantly taking place. The presence of these components determines the redox state of the aquatic environment. It has been proved that under normal conditions the ecological state of aquatic systems is dominated by oxidants, but natural waters also contain a large number of dissolved organic substances that exhibit a reducing character. One of them are thiols, that are substances containing functional group -SH. Thiols are conventionally divided into autochthonous thiols (glutathione (GSH), cysteine), that are substances entering natural waters as a result of metabolism and biochemical processes involving hydrobionts, and allochthonous thiols (thioglycolic acid, thiourea), that are products of anthropogenic impact on aquatic ecosystems. In the surface layers of natural waters the concentration of thiols can decrease not only as a result of interaction with oxidants, but also due to photolysis, which positively influences the formation of the redox state of the aquatic environment. Considering that in natural waters the most frequent thiols are autochthonous thiols, glutathione was chosen as the object of study. Based on bibliography data, transition metal ions can influence the photolysis intensity of thiols. The most common transition metals in natural waters are various forms of iron (10-5M) and copper (10-7M). The kinetics of direct photolysis of glutathione in the presence of Fe(III) and Cu(II) ions was observed by looking at the change of the glutathione concentration, using the Ellman method. A polychromatic lamp model DRT-400 was used as the radiation source. The reaction rates were determined using the Van't Hoff method. From the rate constants and half-life values obtained for the modelled systems shown below it is seen that in the presence of Fe(III) and Cu(II) ions, the photolysis intensity of glutathione is higher and the half-life of glutathione is almost twice as long. Also, from the data obtained, it is seen that the rate constant of photolysis of glutathione in the presence of Fe(III) ions is lower than in the presence of Cu(II) ions, which may be due to the neutral pH value of the modelled systems, at which the Fe(III) ions pass into the suspended state. GSH – hν (k = 2,04 · 10 4 s 1 ; τ 1/2 = 56 min 38 GSH Fe(III) hν (k = 3,36 · 10 4 s 1 ; τ 1/2 = 34 min 23 GSH Cu(II) hν (k = 3,90 · 10 4 s 1 ; τ 1/2 = 29 min 37 s). Also, based on bibliography data, it was hypothesised that the intensity of photochemical transformations of thiols could also be influenced by water mineralization. Based on the fact that most of the natural waters in the Republic of Moldova belong to the hydrocarbonate class of waters, the system GSH – HCO3- – hν was modelled. The rate constant (k = 11,4 · 10-4 s 11) and glutathione half-life (η1 2 = 10 min 8 s) obtained for this system, showed a significant increase of glutathione photolysis intensity. The explanation is that the pH-value also increases with increasing concentration of hydrocarbonate ions and dissociates the thiol group -SH to form thiolate anion (C10H16N3O6S-), which is more reactive than the undissociated group. Another explanation for the increased photolysis intensity of glutathione in the presence of hydrocarbonate ions, according to some bibliography sources, is that hydrocarbonate ions under UV radiation may be electron donors: ν ̅̇. Thus, it has been determined that transition metal ions Fe(III), Cu(II) and mainly water mineralization, particularly hydrocarbonate ions, increase the rate of direct photolysis of glutathione, which is a positive factor in the formation of the redox state of water systems and in the processes of chemical self-purification of natural waters.