About 500 employees were performing routine maintenance at Sasol's Polymers Ethylene plant in Secunda when the blast took place at 9:40 a.m. (0740 GMT), company spokesman Johan van Rheede said.
Preliminary indications were that a gas cloud caused by a leak ignited, causing the explosion about 100 kilometers (62 miles) southeast of Johannesburg, he said.
Sasol's emergency services extinguished the blaze within minutes, van Rheede said, and rescuers were searching the area for a number of people missing after the blast.
More than 100 people were treated for minor to serious injuries, he said. Thirty-two of them were transferred to the nearby Highveld Medi Clinic for treatment.
The Department of Labor and Sasol are investigating the incident.
South African-based Sasol is a liquid fuel provider and chemical producer with operations in 16 countries. The explosion does not affect the company's fuel production, van Rheede said.
