The effect of phosphorous addition to co-precipitated iron/alumina catalysts on HDS activity has been investigated. Two methods of phosphorous addition have been used-phosphate incorporation during the precipitation process, and in situ pre-treatment with PH3/H-2. Phosphate incorporation during the co-precipitation was found to be detrimental to the HDS activity of iron/alumina. An increased reduction temperature of the phosphate containing material, enhanced particle growth during sulphidation combined with a low surface S:Fe ratio contribute to the reduced catalytic activity for total HDS of thiophene measured at 20 bar in the presence of H2S. In situ pre-treatment with PH3/H2 caused a decrease in initial activity, but exposure to HDS reaction conditions resulted in a significantly increased activity, which reached a maximum after 30 h on stream. A similar effect was observed when in situ treatment with PH3/H-2 was done after 40 h on stream. This behaviour may be associated with the occurrence of an optimum S:P ratio.