The influence of initial mixture temperature on the detonability of hydrogen-air-steam mixtures at 0.1 MPa has been investigated experimentally. The detonability of the mixture has been characterized by its cell size. The detonation cell size has been measured in a heated detonation tube using the well-established smoked foil technique. It has been shown that for all the hydrogen-air-steam mixtures tested, increasing the initial temperature decreases the detonation cell size and thus increases the mixture ＇sensitivity＇ to undergo detonation. It has also been shown that the mitigating effect of steam dilution, manifested by an increase in cell size, decreases with increasing initial temperature. A one-dimensional ZND model with full chemical kinetics was used to calculate reaction zone lengths for the mixture conditions tested. A comparison of the measured detonation cell size with the calculated reaction zone lengths indicated that a simple linear relationship between the two does not exist.