This work studied the changes in soil oxidoreductase and hydrolase activities during 360 days of incubation of an arid soil amended with different amounts of organic materials (fresh and composted urban organic wastes). The enzymatic activities were stimulated by such amendments. The soil receiving sewage sludge showed the highest overall activity. The different enzymatic activities rose after amendment then decreased rapidly with time; 360 days after organic amendment, the sail receiving the highest dose showed significantly higher enzymatic activity than the unamended soil. The soil amended with compost showed lower; though more stable, overall enzymatic activity than the soils amended with fresh wastes (municipal solid waste and sewage sludge). The sail receiving organic material to raise its organic matter content by 0.5% did not generally show significant differences from the unamended soil, while the differences produced in the soils receiving organic material to raise their organic matter content by 1.5% were still significant 360 days after amendment.