The coupling relationships between dephosphorization and desulfurization abilities or potentials for CaO-FeO-Fe2O3-Al2O3-P2O5 slags over a large variation range of slag oxidization ability during the secondary refining process of molten steel have been proposed by the present authors as log L-P + 5 log L-S or log CPO43- + log CS2- in the reducing zone and as log L-P + log L-S - 5 log N-FetO or log C-PO4(3-)+ log(CS2-) - log N-FeO in the oxidizing zone based on the ion and molecule 4 coexistence theory (IMCT). In order to further verify the validation and feasibility of the proposed coupling relationships, the effects of chemical composition of the CaO-based slags are provided. The chemical composition of slags was described by three group parameters including reaction abilities of components represented by the mass action concentrations N-i, two kinds of slag basicity as simplified complex basicity (% CaO) / [(% P2O5) + (% Al2O3)] and optical basicity Delta, and the comprehensive effect of iron oxides FetO and basic oxide CaO. Comparing with the strong effects of chemical composition of the CaO-based slags on dephosphorization and desulfurization abilities or potentials, the proposed coupling relationships have been confirmed to not only be independent of slag oxidization ability as expected but also irrelevant to the aforementioned three groups of parameters for representing the chemical composition of the CaO-based slags. Increasing temperature from 1811 to 1927 K (1538 to 1654 degrees C) can result in a decreasing tendency of the proposed coupling relationships. In terms of the proposed coupling relationships, chemical composition of slags or fluxes with assigned dephosphorization ability or potential can be theoretically designed or optimized from its desulfurization ability or potential, and vice versa. Considering the large difference of magnitude between phosphate capacity CPO43- and sulfide capacity CS2-, the proposed coupling relationships 4 between dephosphorization and desulfurization abilities for CaO-based slags are recommended to design or optimize chemical composition of slags.