Carbon deoxidation is a clean steelmaking route, but rapid CO generation can increase gas holdup, raise the melt level, and cause splashing. However, a quantitative link between internal deoxidation kinetics and melt-level rise has not yet been established. Hot-state experiments at 1873 K in a 2 kg crucible under argon with 0.20–1.00 wt.% C were coupled with a dual-pathway kinetic model developed by decoupling free-surface mass transfer from internal CO generation through heterogeneous nucleation at active refractory pores and incorporating an effective reaction depth constrained by hydrostatic pressure. The model reproduced measured oxygen evolution and revealed three deoxidation stages. Internal gas generation initially dominated, contributing 96.6% of the overall rate. As carbon–oxygen supersaturation fell below a critical threshold, the nucleation zone receded upward and free-surface mass transfer became dominant. Within the investigated conditions, the internal apparent deoxidation rate constant during the rapid stage showed an empirical linear relationship with the measured maximum melt-level rise. The critical splash-prevention rate was 4.25 min⁻¹ for the crucible and was estimated to be 2.43 min⁻¹ for a selected 85 t ladle by combining this relationship with the average gas holdup relation and ladle geometry. The present results provide a kinetic basis for assessing splash risk during atmospheric carbon deoxidation.