Arsenic pressures over Ta 3As have been measured using the mass-effusion method in the temperature range 1569 to 2091 K. The least-squares fitted equation for the monatomic arsenic partial pressure is log 10{p( As)/ Pa} = − 23662(K/T) + 1205775 The second-law ΔH o(1829 K) and the estimated ΔH o(298.15 K) for the decomposition reaction of Ta 3As are (453.4 ± 8.0) and (466 ± 14) kJ mol −1 respectively, the errors shown being estimates of total error.