The sulfidation of disk-shaped MoO3 particles with a thickness of 5-10 nm supported on a 5-nm-thick layer of SiO2 on Si(100) in a mixture of 10% H2S in H2 at atmospheric pressure has been studied as a function of temperature. XPS and SIMS indicate the formation of Mo4+OSx at the surface and Mo(IV) oxides (probably H1.6MoO3 or MoO2) in the interior of the particles at temperatures between 20 and 100°C, whereas MoS2 forms at temperatures of 125°C and higher. Sulfur is present in two forms, as S2- and in a second form which is most probably S2 2- or SH-, but not elemental sulfur. The additional sulfur species disappear at temperatures between 150 and 200°C. Rutherford backscattering analysis indicates S:Mo atomic ratios of 1-1.5 at sulfidation temperatures below 100°C and of 2-2.5 above 100°C. It is concluded that the sulfidation of MoO3 to MoS2 proceeds through a Mo(IV) oxysulfide, formed initially at the outside of the particle, and Mo(IV) oxide in the interior of the particle. Sulfidic species are believed to facilitate the reduction of MoO3 to Mo(IV) species at low temperatures.