Officers responded to a report of a woman screaming shortly after 1 a.m. in the 3600 block of Durfee Road, El Monte police said.
Upon arrival, officers found a woman lying in a common driveway with severe burns to her face and body. She was transported to a nearby hospital with life-threatening injuries, police said.
Investigators determined that an unknown suspect poured an unknown type of caustic chemical on the woman before fleeing the area in a vehicle.
The reason for the assault is unknown. An investigation is ongoing.
Anyone with information regarding the crime was asked to contact the El Monte Police Department at 626-258-8635.