The kinetics and reaction mechanisms of hot metal desulfurization using CaO and CaC2 were studied in a well-controlled atmosphere with a lab scale high temperature furnace. The growths of CaS around CaO and CaC2 were measured and compared at 1773 K (1500 degrees C). The parabolic rate constant was evaluated to be 5 x 10(-7) (cm s(-1)) on CaO particles, and 2.4 x 10(-7) (cm s(-1)) on CaC2. The bigger parabolic constant of CaO resulted in more efficient desulfurization. Agglomerates and big CaO particles led to 2CaO center dot SiO2 formation which hindered further utilization of CaO for desulfurization. The 2CaO center dot SiO2 formation was favoured by a high oxygen potential. Since the desulfurization reaction of CaO not only produced CaS but also oxygen, the local oxygen concentration around big CaO particles was higher than around small particles.