Abstract: At approximately 9:30 p.m. on October 5, 1993, operators were attempting to remove sulfur dioxide from a reactor vessel by passing 50 percent sodium hydroxide (NaOH) through the vessel to form an intermediate with the two chemicals. The operator had failed to determine what vapor or gas was in the vessel before startinig the prcess. Therefore, the operator could not determine the amount of sodium hydroxide that needed to be added to the reactor, and sulfur dioxide vapor was released into the air. Employees #1 through #6 experienced symptoms such as headache, sore throat, and tightness in the chest. They were sent to the hospital and released. Violations of 1910.119 were determined to exist and were cited.