Phosphorus-rich waste activate sludge (WAS) has attracted considerable attention as an excellent material for P recovery from sewage. For a satisfactory recovery rate, an efficient release of P from WAS is critical. This paper investigated the effectiveness of four classes of sludge pretreatments, i. e., thermal hydrolysis, sonication, pH adjustment and ozonation, on P release from thickened WAS of an A(2)O wastewater treatment plant (WWTP). Standards, Measurements and Testing protocol (SMT, by European commission), P-31 nuclear magnetic resonance (NMR), excitation-emission matrix (EEM) fluorescence spectroscopy and scanning electron microscopy (SEM) were used to elucidate the evolution of P fractions and their distribution during the pretreatments and the followed anaerobic sludge digestion (only sonicated and thermal hydrolyzed sludge). For the studied WAS (inorganic P content: 74%), the results show that pH adjustment, among the sludge pretreatment methods, was the most effective to release P with the resulting aqueous total P (TPaq) amounting to 30% and 34% of the sludge total P (TPmx) at pH2 and pH12, respectively. The releases of P were not noticeably correlated with those of COD or N, suggesting different sources and mechanisms. The pH adjustment-induced P releases were mostly due to the dissolution of orthophosphate. Anaerobic digestion (AD) was not an effective way to release P from WAS regardless of sludge pretreatments, while pretreatments resulting sludge disintegration significantly improved P mineralization during AD. Because of the difficulty in separating those fine phosphate particles (often observed in WWTP sludge) for recovery, we suggest induction of in situ crystallization of phosphate during AD to be more researched in addition to the conventional re-dissolution - crystallization route.