A new one pot reaction of hydroformylation-cyclotrimerization of cyclopentene 1a and cyclohexene 1b was selectively catalyzed by Rh-6(CO)(16) and H3PW12O40.xH(2)O (HPA-W-12) in THF as a solvent at 40atm (CO/H-2 = 1/1) to give 2,4,6-trisubstituted-1,3,5-trioxanes 3a and 3b as major products along with the corresponding aldehydes 2a and 2b. (C) 2003 Elsevier Science B.V. All rights reserved.