The equilibrium As4(g)=2As2(g) has been studied in detail over solid phase MoAs2 and Mo2As3 using the Knudsen effusion technique combined with a mass spectrometer. A ``see-through'' ion source and a liquid-nitrogen cooled condensation plate were used to eliminate the spurious As4(g) which has been encountered by other investigators. Excellent agreement was obtained between the second- and third-law enthalpies. From equilibrium constants measured between 807 and 1050°K, the following third-law heats were determined: As4(g)=2As2(g), ∆ H0°=53.95± 1.1, ∆ H298°=54.26± 0.7, ∆ H0°(f)[As2(g)]=45.65± 0.7, ∆ H298(f)°[As2(g)]=45.45± 0.7, all in kilocalorie mole-1, the errors being estimates of total error. The results obtained when GaAs, Mo5As4, and Zn3As2 were used as source phases are briefly described. The results of previous investigations of the As4(g)=2As2(g) equilibrium are critically discussed.