Titania-supported iridium catalysts were prereduced in a hydrogen flow at 473 K (IT) and 723 K (HT). Metal particle sizes determined by H-2 chemisorption and by direct observation of metal particles by transmission electron microscopy were quite similar for IT treatment, and close to 3.0 nm. For the HT case, a large difference in particle size between both techniques is obtained, as a consequence of the H-2 chemisorption suppression (SMSI effect). Photoelectron spectra revealed that iridium is in different oxidation states, with a contribution of 19% Irdelta+ and 81% Ir-sigma species for the LT sample and only a slight increase in Irdelta+ species (26%) for the HT catalyst. Further insights into the surface structures developed by IT and HT treatments were derived from the catalytic performance in the vapor-phase hydrogenation of crotonaldehyde. Activity, expressed as turnover frequency, was more than one order of magnitude higher for the HT catalyst than for its IT counterpart. The interfacial metal-TOx moieties, created upon reduction treatment, appeared to be responsible for the increase in activity and in selectivity to crotyl alcohol, via the formation of a [-C=O... surface] sigma-bonded complex (detected by in situ DRIFTS as a band at 1650 cm(-1)) stabilized at the metal-TiOx interface. HT treatment enhances the metal-TiOx contact, which results in an improvement in catalyst performance. The catalysts deactivate slowly with the time of reaction. Two reasons are advanced to explain the catalyst deactivation: (i) the formation of a strongly chemisorbed asymmetric carboxylate (band at 1740 cm(-1)); and (ii) the formation of heavy products with conjugated C=O and C=C bonds (band at 1720 cm(-1)). Both complexes are formed at the expense of the sigma-complex and progressively block the active sites responsible for crotonaldehyde hydrogenation.