A furnace was fabricated to crack phosphine in ultrahigh vacuum. Thermodynamic calculations showed that phosphine should dissociate into the phosphorus dimer to a high degree. Furnace materials were compared by construction of all-tantalum and all-quartz cartridges which slid into the furnace hot zone. Phosphine was found to react with tantalum, but not to any great extent with quartz. The effects of baffles, studied with the quartz cartridges, lowered the temperature at which the cracking efficiencies began to increase and increased cracking efficiencies. The temperature range studied was from 100 ° to 1170 °C.