Biodegradability is a highly desired property as it prevents PEG molecules from accumulating in the environment and, therefore, constrains the exposure component in environmental risk assessment scenarios, irrespective of potential hazard, which is object of investigation of academic research as well as product safety screening studies. Transformation reactions that beak PEG chains, generating lower molecular weight (MW) molecules, are expected to enhance biodegradability: This is because, first, the number of hydroxyl end groups per unit molecular mass increases while the MW decreases, thereby also increasing the probability of exolytic enzymatic breakdown. Second, uptake rates of PEG molecules across microbial membranes into cells -a prerequisite for intracellular metabolic conversion -are expected to increase with decreasing MW. While the dependency of PEG MW on environmental biodegradation has not yet been systematically studied, the published data support faster biodegradation of PEGs with lower MW. For instance, PEG molecules up to 14600 Da showed complete degradation within 20 days in activated-sludge inoculated freshwater media, while PEG molecules with higher MWs required longer incubations for complete biodegradation (i.e., 60 days for PEGs with a number average molecular weight 𝑀 ! """" = 57800 Da). Similarly, PEG molecules with average MW of 400 Da biodegraded in agricultural topsoils within 42-71 days , while molecules with average MW of 4000 Da biodegraded more slowly (extrapolated biodegradation extent of 50% only after estimated 735 days). In sunlit aquatic systems, PEGs may react with photochemically produced hydroxyl radicals (•OH), resulting in chain scissions and thus decreasing MW of the PEGs. The rate of this reaction increases with increasing steady-state •OH concentrations ([•OH]ss) in surface water, which are reported to range from 10 -17 M to 10 -15 M. Major photosensitizers for •OH are nitrate, nitrite, and dissolved organic matter (DOM). The reaction of PEG with •OH is well-studied for engineered systems as •OH are generated in commonly used water treatment processes such as UV/H2O2 treatment and ozonation, which are widely implemented around the world. These studies have shown that PEG chain scissions occur, thereby lowering the average MW and increasing polydispersity index (PDI) of PEGs. By contrast, PEG reactions with environmental •OH and its implications, particularly a hypothesized increase in PEG biodegradability, remain unexplored.