A laboratory experiment compared the rates of nitrogen mineralization from two sludges with varying characteristics. The sludges were mixed with soil at rates of 150, 300 and 600 ppm organic‐N and the mixtures incubated for 13 weeks. The sludge produced by aerobic digestion (Richmond) had mineralization rates of from 36.1 to 60.8% of the organic‐N while the sludge produced by anaerobic digestion, lime addition, and de‐watering (Burlington) had mineralization rates of from 13.7 to 25.2%. The amount of N mineralized from sludge treatments as well as the soil check treatment were well correlated with an autoclaving test for N availability.