Studies on the formation of TiC and Ti(C, N) from hot metal were preformed to determine the conditions required for the formation of a protective layer rich in titanium carbonitride on the refractories in the blast furnace hearth. In situ observations of the formation of TiC and Ti(C, N) from carbon-saturated iron containing 0.07 to 1.3 mass% titanium were made at 1 673 to 1 873 K using a confocal scanning laser microscope equipped with an infrared image furnace. In order to measure the activity coefficient of titanium, the solubility of titanium in carbon-saturated iron was measured and found to be 1.3, 1.0, and 0.3 mass% at temperatures of 1798, 1772, and 1670 K, respectively. The activity coefficient of titanium in carbon-saturated iron is expressed as logf_Ti=–5.41+6890/T. Based on the results the conditions for Ti(C, N) formation and the amount of TiO₂ addition to a blast furnace were calculated. At 1 773 K and 1.0 atm of nitrogen, the minimum titanium in hot metal to form Ti(C, N) was determined to be 0.2 mass%. Under typical blast furnace operation conditions, a titanium load of 3.6–5.2kg TiO₂/tHM is required. It was conclude that in situ observation using confocal scanning laser microscope provided us a unique and reliable technique to study thermodynamic of some molten metal systems.