In this research, investigations were made on the effects of organic substrate types and their load on phosphorus release. Reactors of three different sizes were operated, being fed on five kinds of organic substrates. The quantitative analyses were made on phosphorus release and substrate utilization under anaerobic condition. The molar ratios of the uptaken organic substrate to the released phosphorus were 0.5 with acetate, 0.6 with glucose, 0.8 with glucose/acetate, and 1.2 with glucose/acids, respectively. The phosphorus release was inhibited at the higher organic load than the steady state organic load. Both acetate and acids/glucose enhanced phosphorus release as well as uptake rate, however, the complete phosphorus removal was achieved after the complete microbial adaptation to the environment. With acetate, operation was hampered by the poor sludge settleability and phosphorus was not uptaken enough though the high phosphorus release was effective. With milk/starch, the phosphorus release and uptake were well developed even though phosphorus release was not comparatively high. From these observations, organic substrates, such as glucose seemed to convert into fatty acids after fast biosorption, and concurrent uptake of these acids by excess phosphorus removing bacteria followed. (C) 1997 Elsevier Science Ltd.