Methanol and ethanol are being considered as alternative fuels for diesel engines. One of the key concerns with using alcohol fuels in diesel engines is their poor ignition quality. This work presents the ignition characteristics of methanol and ethanol examined under simulated diesel engine conditions in a constant-volume combustion vessel. The ignition characteristics of isooctane and normal hexadecane (cetane) measured under the same conditions are also included for reference.Results show that to obtain ignition delays and rates-of-pressure-rise suitable for current diesel engine designs, methanol and ethanol require in-cylinder temperatures of about 1100 K at the time of injection. The results also show that the ignition delays of the alcohol fuels are independent of the chamber pressure and are unaffected by the presence of 10% by volume of water in the fuel. Finally, in spite of large differences among the ignition delays of methanol, ethanol, isooctane, and cetane at lower temperatures, their ignition delays converge and reach a limiting value for temperatures above 1100 K. when ignition delay is defined as the time to recover the pressure loss from fuel evaporation.