A thermodynamic model for predicting phosphate capacity C-PO(4)3- of the CaO-based slags over a temperature range from 1750 to 1793 K (1477-1520 degrees C) during dephosphorisation pretreatment of hot metal has been developed based on the ion and molecule coexistence theory (IMCT), i.e. the IMCT-C-PO(4)3- model. The developed IMCT-C-PO(4)3- model has been verified to be valid through comparing with the measured and the predicted ones by four reported C-PO(4)3- models from the literature. Besides the total phosphate capacity C-PO(4)3-, the respective phosphate capacity C-PO(4)3-, i of nine dephosphorisation products as P2O5, 3FeO center dot P2O5, 4FeO center dot P2O5, 2CaO center dot P2O5, 3CaO center dot P2O5, 4CaO center dot P2O5, 2MgO center dot P2O5, 3MgO center dot P2O5 and 3MnO center dot P2O5 can also be accurately predicted by the developed IMCT-C-PO(4)3- model. The formed 3CaO center dot P2O5 accounts for 99.20% of dephosphorisation products comparing with the generated 4CaO center dot P2O5 for 0.80%. The results also revealed that the effects of slag chemical composition including slag oxidisation ability, slag basicity and the comprehensive influence of iron oxides FetO and basic oxide CaO on phosphate capacity C-PO(4)3- were largely different with those on phosphorus partition LP between the CaO-based slags and hot metal. Although C-PO(4)3- and LP are both distinct thermodynamic quantities and related to each others provided that the entire slag system is correctly defined, phosphorus partition LP has much guide meaning on dephosphorisation operation than phosphate capacity C-PO(4)3-. The main interest of measuring phosphate capacity C-PO(4)3- from laboratory experiments can be attributed to obtain phosphorus partition LP through relationship between C-PO(4)3- and L-P.