The energy absorption and decomposition of PH3in a focused CO2laser beam exhibit a threshold nature. The effect of laser frequency, pulse energy, pressure of PH3, and the presence of a foreign gas (neon) on the energy absorption and decomposition threshold were studied. The absorption of radiation increases sharply at threshold and is always accompanied by a visible luminescence. All the experimental facts can be explained in terms of an electron avalanche process, driven by inverse bremsstrahlung absorption of energy from the field of a focused IR laser beam. The simplified phenomenological theory of the dielectric breakdown of gases was applied to the explanation of the threshold phenomena. The decomposition of phosphine was studied as a function of laser frequency, pressure of PH3, and in the presence of foreign gases. The only products of the decomposition found were H2in the gas phase and a solid deposit PHx. The partition of hydrogen between the gas and solid phases depends on the experimental conditions. A reaction scheme is presented which accounts satisfactorily for the experimental facts.