Atomic arsenic was produced in the (4²D) and (4⁴S) states by the flash photolysis of AsCl₃, and the rates of spin-orbit and total relaxation were measured in various gases by kinetic absorption spectroscopy. Xe and most diatomic and polyatomic gases induce the spin-forbidden process with high efficiency. It was suggested that in all cases the transitions occur at the crossing-point of converging potential surfaces. In added oxygen, flash photolysis of AsCl₃ produced the AsO molecule, excited both in respect of vibration and of spin-orbit orientation.