In this study, the effect of feedwater pH (3-11) on phosphorus (P) transformation during the hydrothermal carbonization (HTC) of sewage sludge (SS) was investigated at a temperature range of 200-260 degrees C. The HTC significantly accumulated P in the hydrochar. Different feedwater pH stimulated the transformation of various forms of P. An acidic feedwater pH promoted the transformation of apatite phosphorus (AP) to non-apatite inorganic phosphorus (NAIP), and of organic P (OP) to inorganic P (IP). The NAIP tended to transformation to AP and a small part of the IP was transformed to OP when the SS was treated in a basic environment. The combination of three P analysis methods (chemical extractive fractionation, X-ray powder diffraction (XRD) and energy dispersive spectroscopy (EDS)) showed that metal cations (e.g. Al and Ca) and the pH played important roles in the transformation of different forms of P during the HTC of the SS.