In this study, the in vitro bioaccessibility (IVBA) of lead (Pb) in phosphate-amended Pb-contaminated soil was assessed using a variety of IVBA assays with an overarching aim of determining whether changes in Pb IVBA were congruent to those observed for Pb relative bioavailability (RBA) determined using an in vivo mouse assay. Amending soil with phosphoric acid or rock phosphate resulted in changes in Pb speciation, however, varying Pb IVBA results were obtained depending on the methodology utilized. In addition, IVBA assays influenced Pb speciation as a consequence of interactions between dissolved Pb and unreacted phosphate arising from the amendment or from assay constituents. When the relationship between Pb RBA and IVBA was assessed, a comparison of treatment effect ratios (Pb RBA or IVBA in treated soil divided by Pb RBA or IVBA for untreated soil) provided the best in vivo-in vitro correlation particular for SBRC (r2 = 0.83) and IVG (r2 = 0.89) intestinal extraction. For these assays, the slope of the lines of best fit were close to 1 (1.12, 0.82; SBRC, IVG intestinal extraction respectively) with small y-intercepts (0.09, 0.08 respectively) indicating that the efficacy of phosphate amendments for reducing Pb RBA may be predicted using IVBA assays.