One main biotransformation product (BTP) was formed from quetiapine (QUT).
Six phototransformation products (PTPs) of QUT were identified for the first time.
QUT was not readily biodegradable; photolytic mixture was not better biodegradable.
Several different BTPs were formed from PTPs in the photolytic mixture.
QUT showed cytotoxic activity in bacteria which was not reduced by photo-treatment.