Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630 degrees C and 195 MPa, was performed in Ar/O-2 (dry oxidation), Ar/O-2/H2O (wet oxidation), and Ar/H2O (H2O oxidation) at 900 degrees-1200 degrees C. The weight change measurements of the TiC specimen showed that the dry, wet, and H2O oxidation at 850 degrees-1000 degrees C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100 degrees and 1200 degrees C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO2 scale consisting of many thin layers, about 5 mu m thick, containing many pores and large cracks, while H2O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO2 and H-2 with weight change measurement was simultaneously followed by heating the TiC to 1400 degrees C in the three atmospheres. Cracking in the TiO2 scale accompanied CO2 evolution, and the H2O-containing oxidation produced a small amount of H-2. A piece of single crystal TiC was oxidized in O-16(2)/(H2O)-O-18 to reveal the contribution of O from H2O to the oxidation of TiC by secondary ion mass spectrometry.