Even appropriately used, pesticides can enter the surface and groundwater by several routes where photochemical degradation along with biotic processes contributes to their fate, resulting sometimes in the formation of stable transformation products (TPs). Yet, little is known about S-metolachlor (SM) transformation in the aquatic environment. Furthermore, commercial formulation of a pesticide might have different physical and biological properties compared to its pure grade. The present study assessed the biodegradability of the pure SM and its commercial product Mercantor Gold(®) (MG) by employing two OECD biodegradation (301D, F) tests. Photolysis in water was investigated by using a Xe lamp. Subsequently the biodegradability of the photolysis mixtures was examined. The primary elimination of SM was monitored and structures of its TPs were elucidated by HPLC-UV-MS/MS. Additionally, a set of in silico prediction programs was applied for supporting analytical results and toxicity assessment of SM and TPs. S-metolachlor and Mercantor Gold(®) were not biodegraded. HPLC-UV analysis showed higher elimination of SM in MG compared to pure SM during photolysis. A total of 10 photo-TPs of SM and MG were identified. According to MS data and in silico predictions, chemical structures were proposed for all found photo-TPs. Likewise for the parent compounds, no biodegradation has been observed for their photo-TPs. However, in the 301F test new bio-TPs have been generated from photo-TPs which were observed for the first time according to authors' best knowledge. The results suggest that the MG formulation does not affect the biodegradation process, but it influences the photolysis efficiency and potentially might result in faster formation of TPs in the environment. This study also demonstrates that photo-TPs can be further transformed into new products due to bacterial activity in the water phase. Moreover biotransformation might lead to an increased toxicity compared with the parent compound.