The distribution ratio of phosphorous between the CaO–CaF₂ (–SiO₂) flux and SiMn alloy melts at 1823 K was measured under strongly reducing atmosphere. Furthermore, thermodynamic and kinetics analyses were carried out for the environmental stability of reducing refining slags containing Ca₃P₂ under wet cooling conditions from the effect of slag composition on the evolution of PH₃ (phosphine) gas. The distribution ratio of phosphorous between the CaO–CaF₂ (–SiO₂) flux and SiMn metal phases increased with increasing CaO concentration in the flux, followed by a constant value. The composition for the saturating distribution ratio of phosphorous was in good accordance to the saturation content of CaO in the CaO–CaF₂ flux at 1823 K. When the Vee ratio (= CaO/SiO₂) of the dephosphorization slag was greater than about 1.35, the lime and dicalcium silicate phases precipitated during solidification, resulting in an increase in the evolution rate of PH₃ gas under wet and dry conditions due to an increase in the reaction area. However, when the Vee ratio of the slag was lower than about 1.35, fluorite, cuspidine, and wollastonite phases precipitated from the phase diagram, resulting in less amount of PH₃ evolution during cooling because the reaction between Ca₃P₂ and H₂O was restricted to the surface of bulk slag.