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The sulphidation of two copper-silver alloys containing 5 and 10 wt.% silver (Cu-5Ag and Cu-10Ag, respectively) was investigated over the temperature range 550-750 degrees C in H-2/H2S mixtures under sulphur pressures below the dissociation of Ag2S. External scales, composed mainly of cuprous sulphide, often containing some (Cu,Ag)(2-x)S islands, formed on both alloys. The corrosion kinetics were significantly affected by the detachment of the outer scales at temperature, as a consequence of the significant volume increase due to sulphidation. After scale detachment, sulphidation proceeded through a dissociative transport of sulphur in voids between the outer sulphide layer and the alloy substrate, yielding an abrupt decrease of the corrosion rate in all cases and modifying the composition of the reaction products in the case of Cu-10Ag. An increase of the volume fraction of the silver-rich solid solution in the subscale region with respect to the original alloy was observed in all cases. The silver islands coalesced to a continuous subscale layer only in the sulphidation of Cu-5Ag at 650 and 750 degrees whereas, in all the other cases, they remained as particles in a Cu2S matrix. The peculiar corrosion behaviour of these alloys is attributed to the low solubility of silver in copper. (C) 1999 Elsevier Science Ltd. All rights reserved.