Density functional calculations show that the "magic" structures of titanium carbide nanoparticles display an unexpected high reactivity toward CO, NH3, and H2O when compared to the bulk TiC(001) surface. In spite of the large C/Ti ratio in Ti8C12, our results show that the bulk surface is much more tightly bonded than the nanocluster, and thus the nanocluster has a much higher chemical activity. (C) 2003 American Institute of Physics.