The paper presents a study on the decarburization of pig iron droplets in synthetic BOF slag. The effects of droplet size and slag composition were studied. The results show evidently that the decarburization is very fast in general. One gram of pig iron is mostly decarburized within one minute. The reaction is shown to be faster when many small droplets were employed instead of one big droplet with equal total mass. This is explained by the bigger interfacial area between the liquid slag and the pig iron of the many small particles. It is also interesting to see that the decarburizing reaction in the slag having lower dynamic viscosity and higher FeO activity is slower than in the slag with higher viscosity and lower FeO activity. The slower reaction could be explained by the longer incubation time in this slag.