Phosphine (PH3) is a highly reactive and toxic gas. Prior experimental investigations of PH3 pyrolysis reactions have included only low-temperature measurements. This study reports the first shock-tube measurements of PH3 pyrolysis using a new PH3 laser absorption technique near 4.56 mu m. Experiments were conducted in mixtures of 0.5% PH3/Ar behind reflected shock waves at temperatures of 1460-2013 K and pressures of similar to 1.3 and similar to 0.5 atm. The PH3 time histories displayed two-stage behavior similar to that previously observed for NH3 decomposition, suggesting by analogy that the rate constant for PH3 + M reversible arrow PH2 + H + M (R1) could be determined. A simple three-step mechanism was assembled for data analysis. In a detailed kinetic analysis of the first-stage PH3 decomposition, values of k(1,0 )were obtained and best described by (in cm(3).mol(-1.)s(-1)) k(1,0) = 7.78 x 10(17 )exp(-80,400/RT), with units of cal, mol, K, s, and cm(3). Agreement between the 1.3 and 0.5 atm data confirmed that the measured k(1,0) was in the low-pressure limit. Agreement of the experimental k(1,0) with ab initio estimates resolved the question of the main pathway of PH3 decomposition: it proceeds as PH3 reversible arrow PH2 + H instead of PH3 reversible arrow PH + H-2.