Iron droplets can be entrained into the slag phase when gas bubbles pass through the molten iron/slag interface. The entrainment phenomena occurring during passage of single bubbles through the interface were investigated by using in-situ X-ray transmission (fluoroscopic) techniques and by collecting and measuring the entrained iron droplets. The effects of bubble size, interfacial tension and viscosity of the slag on the iron droplets entrainment were investigated. The results showed that the mass of the entrainment increased with the increasing bubble size, but decreased with increasing interfacial tension and slag viscosity. A dimensionless equation was proposed to describe the relation between the entrainment of metal into slag and the above-mentioned parameters. This information can be used to optimize heat and mass transfer processes e.g. in ladle metallurgy and to reduce the financial loss due to metal discard within the slag.