Cleanliness of steel is mainly determined by the presence of oxide inclusions. The reoxidation of liquid steel before, during and after ladle treatment at the Sidmar, steel plant was quantified for three steel grades, as it is a potential source of inclusions. A mass balance calculation was performed to compare the expected and measured steel composition. The total reoxiclation was calculated from the reactions of the dissolved elements. A dissimilarity in behaviour of the steel grades was found. The resultant compositions of the slags were calculated and compared with measured compositions. Some differences between computed slag compositions and average measured values are explained. Finally, the relation between the reoxiclation and internal cleanliness at the end of ladle treatment was examined.