The effect of C source on N losses by volatilization during composting was measured using four bulking agents, each at three humidity levels and composted in duplicate under passive and active aeration. The bulking agents were pine shavings alone and corrected with soybean, chopped grass hay alone and corrected with urea, long (unchopped) wheat straw and chopped oat straw. The readily available C of each bulking agent was determined by analyzing for BOD5. In 105 l laboratory vessels, the bulking agents were mixed with liquid swine manure and tap water for a C/N of 20 and three humidity levels of 60%, 65% and 70%. While being aerated actively or passively, the mixtures were composted for 21 days. Their initial and final C and N contents were measured to conduct a mass balance analysis and calculate C and N losses. C and N losses were compared to bulking agent BOD5. N losses were compared to C losses. The humidity level and aeration regime had no effect on C and N losses but the N losses were correlated to C losses and only the C losses could be correlated to the BOD5 of the bulking agent. Thus, the N losses are related not only to the availability of C but also to the extent of composting. A relationship established between N and C losses indicated that 85% of the initial total N of the compost was available for microbial degradation and that 70% of the available C was lost as CO2 during the immobilization process.