Abstract: Employees #1 and #2 and another fire fighter were in vehicle #1, responding to an emergency alarm report of an odor along the highway near the waterfront piers. The fire fighters were warned that the odor might be chlorine from a tanker truck. A faint chlorine odor was smelled. Vehicle #1 and HazMat vehicle #2 staged a command at the entry to Pier Drive, where people were evacuating their parked vehicles. Vehicle #1 was instructed to investigate the area windward. The vehicle rounded a curve where the chlorine odor was stronger and a damaged roadside fence of a chemical company was observed. No vapor, fume, or smoke cloud was visible. Vehicle #1 stopped. Immediately, the fire fighters were overcome by an invisible chlorine gas cloud; they choked and could not breathe. The driver struggled to put vehicle #1 in first gear, then drove until the fire fighters could breathe. Employees #1 and #2 were hospitalized and treated for chlorine inhalation. Two fire fighters without SCBA walked back to a point 15 to 25 yards from the damaged fence where the chlorine odor was intense. This allowed them to verify the location where the HazMat vehicle #2 team was needed.