The shock load effect of heavy metals (Cu (II)) on the behavior of poly-phosphate-accumulating organisms (PAOs) was investigated with respect to the transformations of poly-P, intracellular polyhydroxyalkanoates (PHAs) and glycogen. The PAOs biomass was exposed to different concentrations of Cu (II) at various pH and biomass levels. The results showed that when the mixed liquor suspended solid (MLSS) concentration was 2500-4000 mg/L, the P removal was not adversely affected by spiking with 2 mg Cu2+/L; however, it deteriorated completely after a Cu (II) shock concentration of 4 mg/L Nevertheless, the tolerance of PAOs biomass to Cu (II) shock could be enhanced by increasing the MLSS. Moreover, in the presence of 2 mg Cu2+/L, the P removal efficiency was highest at an initial pH of 6.2 and lowest at an initial pH of 6.9, indicating that the Cu inhibitory effect was reduced by increasing the pH to 7.6. The inhibition by Cu (II) was related to the transformation of intracellular storage compounds of PAOs. Specifically, poly-P degradation might be inhibited, which reduced the energy available for PHA production and eventually led to poor P removal. (C) 2010 Elsevier B.V. All rights reserved.