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Phosphorus (P) availability critically limits the productivity of tropical forests growing on highly weathered, low-P soils. Although efforts to incorporate P into Earth system models (ESMs) provide an opportunity to better estimate tropical forest response to climate change, P sorption dynamics and controls on soil P availability are not well constrained. Here, we measured P and dissolved organic carbon (DOC) sorption isotherms on 23 soils from tropical Oxisol, Ultisol, Inceptisol, Andisol, and Aridisol soils using P concentrations from 10 to 500 mg P L-1, and DOC concentrations from 10 to 100 mg DOC L-1. Isotherms were fit to the Langmuir equation and parameters were related to soil characteristics. Maximum P sorption capacity (Qmax) was significantly correlated with clay content (rho = 0.658) and aluminium (Al)- or iron (Fe)-oxide concentrations (rho = 0.470 and 0.461 respectively), and the DOC Qmax was correlated with Fe oxides (rho = 0.491). Readily available soil characteristics could eventually be used to estimate Qmax values. Analysis of literature values demonstrated that the maximum initial P concentration added to soils had a significant impact on the resultant Qmax, suggesting that an insufficiently low initial P range could underestimate Qmax. This study improves methods for measuring P Qmax and estimating Qmax in the absence of isotherm analyses and provides key data for use in ESMs.