Cotton Howell, director of York County Emergency Management, said officials believed the two men were performing maintenance in a decommissioned storage tank at Resolute Forest Products, when an unidentified chemical leaked into the tank.
One of the men, Samir Storey, 39, was pronounced dead at the scene, York County Coroner Sabrina Gast said.
The unidentified second worker was pulled from the tank and was treated for injuries, The (Rock Hill) Herald reported.
The victims were not Resolute Forest Products employees, but contractors with Tradesmen International, a construction labor support company, Resolute spokeswoman Debbie Johnston said.
The federal Occupational Safety and Health Administration, which investigated a chemical spill at the plant in May and determined no OSHA policies were violated, was still determining if they would investigate Tuesday's accident, company spokeswoman Lesia Kudelka said.