Picosecond infrared-infrared pump-probe experiments in the OD stretching region were carried out on dehydrated and multilayer water-adsorbed gamma -Al2O3. For the dehydrated gamma -Al2O3, transient bands assigned to the bleaching and hot bands of the OD stretching mode of isolated surface hydroxyl groups were observed, and the population lifetime (T-1) of the vibrational excited state (nu = 1) of the mode was 200 +/- 10 ps at 293 K. The characteristic temperature dependence of T-1 is indicative of a seven-phonon process. For the wateradsorbed gamma -Al2O3, the transient bleaching and transient hot bands with a lifetime of 10-12 ps were observed by excitation of the absorption peak at 2630 cm(-1). In addition to these bands, weak transient bands were observed in the low-frequency region. These results are considered evidence of the existence of isolated water molecules on the gamma -Al2O3 surface. Pure dephasing is also discussed based on the bandwidth of the observed transient bands.