The one atmosphere isobaric phase relations have been determined for the system As-Sb, using a simple experimental arrangement where a low melting temperature alkali halide melt acts as a reaction vessel. The one atmosphere univariant temperature, where a liquid of composition 45.5 mol % As, plus a solid of composition 62 mol % As and a gas of composition 89 mol % As coexist, is 666 degrees + or - 3 degrees C. The one atmosphere sublimation point of As was measured as 611 degrees + or - 2 degrees C.Arsenic and antimony are hexagonal isostructural compounds with a complete solid solution series between them. The a cell edge and volume of the unit cell vary linearly with composition in mol percent. The c edge changes in a markedly nonlinear manner with composition. The measured unit-cell parameters at 25 degrees C are:As, a = 3.760 + or - 0.001 Aa, c = 10.555 + or - 0.003Aa, V = 129.2Aa 3 ;Sb, a = 4.310 + or - 0.001 Aa, c = 11.279 + or - 0.003Aa, V = 181.4Aa 3 .