Abstract In order to understand the behavior of arsenic and phosphorus in volcanic environments, ejecta and volcanic gases collected at Mt. Usu were analyzed and the differentiation of the two elements was investigated. Results of analyses show that phosphorus is depleted in the gas phase while it is abundant in the rock. On the other hand, arsenic is found to have a high volatility comparable to that of a major component of active volcanic gases like chloride. The apparent partition coefficient (As/P) gas/(As/P) rock is calculated to be in the range from 10 4 to 10 5. In order to examine the existing state of P and As in magmatic gases, the fugacity ratios among chemical species of P and those of As are calculated. It is suggested that P exists overwhelmingly as PO 2, while As exists as As 4 under the Usu magmatic conditions; temp.; 1200 K, f H 2O : 400 atm, f o 2 : 400 atm etc. On the assumption that f PO 2 is controlled by the equilibrium with the mineral assemblage: Ca 3(PO 4) 2 + SiO 2 + CaSiO 3 (in orthopyroxene), the calculated P content is 10 6 times less than that measured in the condensates. Several possible causes of this discrepancy are discussed. The discrepancy may be due to: (1) the admission of phosphorus from wall rock through interaction of ascending volcanic gases with wall rock if apatite is present in the magma: and (2) the formation of PO 2 exclusively from PO 4 3− ion in the magma with the absence of apatite though thermodynamic estimation of f PO 2 is not possible. An absolute value of f As 4 cannot yet be obtained because of the lack of thermodynamic data of arsenic-bearing minerals. The difference in the behavior of arsenic and phosphorus in volcanic environments seems to be closely related with the redox properties of the two elements.