In MRI-based bone mineral assessment, pixel intensity of the bone is compared against that of the sample to estimate the 31P density after correcting for their relaxation properties. Knowledge of bone 31P relaxation times is therefore crucial. Using saturation-recovery spectroscopy and ZTE-PETRA, T1, T2* and density of bone 31P in healthy subjects (26 to 76 y/o) were measured yielding 36.8±1.4s, 196.9±10.1μs and 6.10±0.62 mol/L. Measured T1 and T2* errors are expected to be within 6% and 15%, resulting in an error of quantified [31P] of ≤ 6.3%. The small inter-subject variations may therefore obviate the need for individual T1 measurements.