The effect of the bulking agent on acid and alkaline phosphomonoesterase (PME) activities and beta-D-glucosidase activity was monitored during the composting of cattle and pig manure. The mixture of manures was co-composted with chopped barley straw (MS composting) and with Sphagnum peat (MP composting) in a continuously rotating horizontal-drum composter (a week) in three successive series and subsequently cured in heaps for three months. Enzyme activities were high and varied widely in both the MS and the MP starting materials. All the activities declined sharply after three days' active processing in the drum and increased again during the early curing, but showed a decreasing trend after that. By the end of the curing the acid and alkaline PME activities and the rank order of the enzyme activities were significantly different in the MS and MP composts. The acid/alkaline PME/glucosidase ratios(dm) were 1:4:1 and 3:3:1, respectively. Thus, the choice of bulking agent strongly affected the potential capacity and property for mineralization of organic phosphorus in manure composts, but did not affect glucosidase activity, although this is one of the key enzymes in the decomposition of cellulosic plant materials like straw.