Abstract The equilibrium vapour pressures of arsenic over InAs(c) and GaAs(c) have been studied using the Knudsen-cell mass-spectrometric technique. Appropriate corrections were made for the solubility of arsenic in the liquid metals Ga and In which were produced in the decomposition. The following enthalpies of decomposition were derived: 2 GaAs(c)=2Ga(c)+As 2(g); δH°(298.15 K)=(89.7±1.2) kcal th mol −1; 2 InAs(c)=2In(c)+As 2(g); δH°(298.15 K)=(77.7±1.2) kcal th mol −1; The errors assigned are estimates of total error. Enthalpies of formation of GaAs(c) and InAs(c) were also derived. From measurements of the partial pressures of both As 2 and As 4 over InAs(c), the enthalpy of dissociation of As 4 was also determined. As 1(g)=2As 2(g); δH°(298.15 K)=(54.4±1.5) kcal th mol −1 . This is in excellent agreement with the result obtained previously when MoAs 2(c) and Mo 2As 3(c) were used as sources.