Inhalation of the chemical could lead to death, said Lieutenant Colonel Katlego Mogale.
She said it was in a Krome Metal Chemicals truck which was hijacked in Midrand on Saturday, and she urged people with information about its whereabouts to contact the police.
Mogale said the chemical was toxic if swallowed or if it came into contact with the skin. It could skin and eye irritation, and burns. Even brief exposure to its gas was toxic or extremely irritating.
She said it targeted the central nervous system, eyes, kidneys, liver, lungs, skin and heart.
"Medical symptoms include extreme irritation of eyes and mucous membranes, including burning and tearing; respiratory failure, death," she said.
Mogale said investigations were continuing.