Although the dephosphorization kinetics of bloated metal droplets reacting with oxidizing slag have been studied in detail in the authors’ laboratory, the mechanism of reaction for slags in a basicity range typical of steelmaking, has been sparsely reported. The current study employed a high temperature furnace equipped with X-ray fluoroscopy to observe the bloating behavior of droplets and tracked dephosphorization kinetics by quenching and analyzing droplets after different reaction times. The mechanism of reaction between bloated metal droplets and slag was studied at 1 923 K for slags with basicity range C/S=2.56. The rate and extent of dephosphorization was found to be greater in CMS slags compared to CAS slags due to the faster mass transport and a larger thermodynamic driving force. The kinetic analysis showed that the reaction proceeded in two distinct stages, a fast initial stage followed by a slower stage. The km during the first stage of dephosphorization was at least 8 times higher than that during the second stage. This is proposed to be due to a higher internal CO generation rate during the initial stage which increases the rate of surface renewal. The effect of TiO2 on dephosphorization kinetics was also investigated in terms of thermodynamic driving force.