The growth yields (Y-obs) are greater under substrate-limited conditions than those under substrate-sufficient conditions in continuous cultures. This indicates that the excess substrate should cause uncoupling between anabolism and catabolism, which leads to energy spilling. Although the uncoupling between anabolism and catabolism has already been recognized in the microbiology literature, how to quantitatively describe such uncoupling remains unclear. Based on a balance on substrate reaction, a growth yield model was developed in relation to residual substrate concentration for substrate sufficient continuous cultures. On the basis of that yield model, the concept of an uncoupling coefficient between anabolism and catabolism is defined in this work. A model describing the effect of the residual substrate concentration on the uncoupling coefficient of anabolism to catabolism is proposed. This model agrees very well with literature data.