Ensuring accurate total phosphorus (TP) quantification in sludge is essential for assessing P recovery potential and optimizing wastewater management. This study addresses two critical aspects: the reliability of TP measurement techniques and the influence of sludge composition on P solubilization during thermal hydrolysis (TH). First, the challenges associated with TP determination in untreated and post-TH sludge were evaluated. The Standards in Measurements and Testing (SMT) extraction protocol—a simple and widely used method often applied to biological sludges—was found to underestimate TP by up to 34 % in matrices such as post-TH sludge, compared to more robust digestion methods. While previous studies have reported qualitative discrepancies among extraction methods, this work quantifies the bias under TH conditions through a benchmark combining mass balance analysis and Monte Carlo simulations. Using the proposed methodology, results highlight the importance of selecting appropriate TP quantification methods tailored to sludge characteristics to ensure accurate recovery assessments. Second, the influence of biological sludge sources on P release during TH was evaluated using two sludge samples: one from a full-scale wastewater treatment plant (S1) and another from a lab-scale sequencing batch reactor (S2). Results showed significant differences in P release percentages. Soluble reactive P recovery exceeded 78 % in S2 but remained below 25 % in S1. Solution 31P NMR showed that both sludges were initially rich in polyphosphate (poly-P) (~80 %), yet after TH poly-P in S1 was almost completely hydrolyzed to orthophosphate, which remained bound in the residual solid. In contrast, S2 produced mainly pyrophosphate and retained some poly-P. Elucidation of the precise link between poly-P structure, its interaction with metals, and the resulting P release efficiency warrants further high-resolution analysis.