Furan, C4H4O, has been observed from acetaldehyde over beta -UO3 during temperature programmed desorption (TPD) with a high yield (ca. 40%) at low surface coverage. At high surface coverage crotonaldehyde, CH3CH=CHCHO (formed by beta -aldolisation of acetaldehyde), was the most dominant product. Flow experiments at P = 30 atm have indicated that one can achieve high reaction selectivity to furan. The catalyst deactivated, however, after a few hours of time on stream. X-ray diffraction analyses have shown that beta -UO3 has been reduced to a mixture of alpha -U3O8 and UO2. The catalysts could be regenerated by gas-phase O-2 at 673-773 K, 30-40 atm for 2 h. The comparison between furan formation from acetaldehyde to that from ethylene as well as fi om ethanol [J. Catal. 184 (1999) 553; Stud. Surf. Sci. Catal. 110 (1997) 265] over U oxides indicates the following. (1) beta -UO3 was far more active than alpha -U3O8 (2) Acetaldehyde gave the highest reaction yield. (3) The reaction appears to be driven by the high oxidation state of U cations (U+6) as well as the potential presence of these U+6 cations in a sixfold coordination environment, i.e. containing two vacancies to accommodate the coupling of two C2 molecules to the C4 furan product. (C) 2000 Elsevier Science B.V. All rights reserved.