Pt/TiO2 is an active and selective catalyst for single stage water gas shift reaction. However, the catalyst deactivated with time on stream. Detailed studies of fresh, used, and reactivated Pt/TiO2 catalysts were carried out using kinetic (steady state and transient) and in situ IR spectroscopic methods in order to understand the mechanism of deactivation. The loss of Pt surface area was the cause of Pt/TiO2 deactivation, exclusively. Pt sintering occurred mainly due to the presence of traces of formaldehyde formed under WGS reaction conditions by reaction of H2 and CO. Details concerning the deactivation behavior are discussed.