BRIDGEVIEW, Ill. (AP) _ A fiery explosion at a chemical plant early Monday critically injured two employees and forced about 300 people from their homes during the night, authorities said.

Houses near the Unocal Corp. plant in suburban Chicago were evacuated for about two hours when thick black fumes drifted through the area, police said.

''We heard the explosion and jumped out of bed like a spring,'' said Sophie Robish, who lives two blocks from the plant.

Authorities had not determined if the fumes were toxic.

''I just got done reading and was getting ready for bed when I heard this very loud boom,'' said Katie Dyrda, another nearby resident.

She said police asked residents to leave their homes about an hour after the 1 a.m. explosion. Mrs. Dyrda said she grabbed her 3-year-old son, Sean, and headed for a nearby community center set up for the evacuees.

Investigators from the U.S. Environmental Protection Agency and the Illinois EPA were at the scene, about 13 miles southwest of Chicago, trying to determine the cause of the blaze, said Chuck Corkery of the Cook County Emergency Services and Disaster Agency.

The polymer manufacturing plant makes latex, an adhesive that binds materials in paint together. The blast apparently occurred in an area where combustible materials are used, especially designed with reinforced walls to minimize damage from an explosion, said John Maher, manager of field services at the Unocal's chemical division in suburban Schaumburg.

''All we know is that they were in the process of feeding chemicals into a reactor, and it exploded,'' said Police Sgt. Thomas Frale.

Two employees were in critical condition at Loyola University Medical Center in nearby Maywood, said nursing supervisor Diane Geeting. Two other injured employees were in good condition at another hospital.