Natural organic matter (NOM) often forms coatings on minerals. Such coatings are expected to affect metal–ion sorption due to abundant sorption sites in NOM and potential modifications to mineral surfaces, but such effects are poorly understood in complex multicomponent systems. Using poly(acrylic acid) (PAA), a simplified analog of NOM containing only carboxylic groups, Pb(II) and Zn(II) partitioning between PAA coatings and α-Al2O3 (1–102) and (0001) surfaces was investigated using long-period X-ray standing wave-florescence yield spectroscopy. In the single-metal–ion systems, PAA was the dominant sink for Pb(II) and Zn(II) for α-Al2O3(1–102) (63% and 69%, respectively, at 0.5 μM metal ions and pH 6.0). In equi-molar mixed-Pb(II)–Zn(II) systems, partitioning of both ions onto α-Al2O3(1–102) decreased compared with the single-metal–ion systems; however, Zn(II) decreased Pb(II) sorption to a greater extent than vice versa, suggesting that Zn(II) outcompeted Pb(II) for α-Al2O3(1–102) sorption sites. In contrast, >99% of both metal ions sorbed to PAA when equi-molar Pb(II) and Zn(II) were added simultaneously to PAA/α-Al2O3(0001). PAA outcompeted both α-Al2O3 surfaces for metal sorption but did not alter their intrinsic order of reactivity. This study suggests that single-metal–ion sorption results cannot be used to predict multimetal–ion sorption at NOM/metal–oxide interfaces when NOM is dominated by carboxylic groups.