The literature on the experimentally measured arsenic activities in molten copper alloys was reviewed. It was found that the arsenic activity coefficients at infinite dilution in molten copper, γAs0, differ by one to three orders of magnitude in the temperature range 1273 to 1423 K. Such discrepancies may be due to different arsenic thermodynamic and vapor pressure data used by different authors. Based on more accurate data for arsenic vapour species, vapour phase equilibria at various temperatures have been calculated. Tetramer As4 is the predominant species in the vapour phase saturated with solid or liquid arsenic. When arsenic vapour, saturated previously with solid arsenic, is heated to temperatures above 1273 K, As2 becomes the predominant species. Recalculation of the arsenic activities in molten copper from literature data shows that RTlnγAs0 is fairly constant and has an average value of -69 229 J/mol in the temperature range 1273 to 1423 K.