Although strong in vivo-in vitro correlations (IVIVCs) between relative bioavailability (RBA) and bioaccessibility of soil Pb were well reported, knowledge on the fractions of bioaccessible Pb in simulated gastrointestinal (GI) fluids that are available for absorption into the systemic circulation is limited. Here, Pb-RBA in 14 Pb-contaminated soils were assessed using an in vivo mouse bioassay and compared to Pb bioaccessibility by the gastrointestinal phase of the UBM (Unified Bioaccessibility research group of Europe (BARGE) Method) in vitro assay with and without 0.45 μm filtration of GI fluid. Results showed good IVIVC between Pb-RBA and Pb bioaccessibility without filtration (r ² = 0.62), while Pb bioaccessibility with filtration provided a poor correlation with Pb-RBA (r ² = 0.16). This suggested that besides dissolved Pb ions, Pb-complexes formed in the UBM gastrointestinal fluid might also contribute to bioavailable Pb. To ascertain this, DGT (diffusive gradients in thin-films) devices which can measure both Pb ²⁺ ions and labile inorganic and organic Pb-complexes were introduced to the UBM fluids to measure Pb DGT-bioaccessibility, which showed strong correlation to Pb-RBA (r ² = 0.71). With increasing diffusive gel thickness which could enhance release of Pb ions from Pb-complexes, Pb DGT-bioaccessibility increased by 3.4-5.7 times, while inclusion of dialysis membrane within DGT devices significantly decreased Pb DGT-bioaccessibility by inhibiting diffusion of Pb complexes to binding gel. These results confirmed the contribution of Pb-complexes to Pb bioavailability, providing new insights to Pb bioavailability.