To analyze the arsenic behavior in the copper smelting process, the phase relations and the arsenic activity in the miscibility gap of Cu–Fe–S–As system with arsenic as a minor element have been determined at 1473 K by the quenching method and the double Knudsen cell-mass spectrometric method, respectively. The experiments have been conducted for the charges in the miscibility gap with mass%Fe/mass%Cu ratios of 0, 0.023, 0.059, 0.098 and 0.131 with varying arsenic content. The activity measurements indicate that the arsenic activities in both the metal and matte phases show extremely negative deviation from the ideal behavior. The Raoultian activity coefficients at infinite dilution in the metal and matte phases are found to be almost constant against the charges in the miscibility gap and they are 4.3×10⁻³ and 5.5×10⁻² for the metal and matte phases, respectively. The vapor pressures of predominant As, As₂ and AsS species in the gas phase equilibrated with the immiscible solutions have been calculated on the basis of the determined activity coefficients and they are very small at less than 1 Pa even when the arsenic content in the metal phase is increased up to 10 mass%.