Ultrafast detonations having steady-state velocities greater than predicted by the Chapman-Jouguet (C-J) and the Zeldovich-von Neumann-Doring (ZND) theories are predicted by direct simulations of detonation waves in gaseous reaction systems. The simulations are made using Bird's direct simulation Monte Carlo method which produces the full details of the coupled gas-dynamic and reaction effects as well as temperature, velocity, density, pressure, and species profiles for the detonation waves. The systems are simplified and clarified by restriction to one-dimensional flow with the reaction A+M-->B+M having variable energy release and rate characteristics. For a slow reaction the reaction and shock regions are separated and the detonation wave proceeds at the forward speed of sound in the burned gas as predicted by the C-J and ZND theories. For a very fast reaction the reaction and shock waves overlap, the assumptions required for the C-J and ZND theories are no longer valid, the von Neumann spike is reduced, and the detonation velocity exceeds that predicted by the C-J and ZND theories to produce an ultrafast detonation. (C) 2003 American Institute of Physics.