The adsorption and surface reactions of formaldehyde were studied on TiO2 and Rh/TiO2 catalysts at 300-473 K by Fourier-transform infrared spectroscopy (FTIR) and mass spectrometry (MS). FTIR bands due to molecularly adsorbed formaldehyde, formate, and formic acid were detected. The dominant surface species was dioxymethylene (DOM, H2CO2(a)) species, a part of which underwent surface polymerization resulting in polyoxymethylene (POM) on the surface. At higher temperatures molecularly adsorbed formaldehyde and DOM surface species decompose producing mainly H-2 and CO in the gas phase. The amounts of H2 and CO were proportional with the Rh content of the catalysts. The quantity of ethylene (also formed in the above decomposition), however, changed inversely with the increase of Rh content. (C) 2004 Elsevier Inc. All rights reserved.