Abstract The one-pot hydroformylation–cyclization of allylbenzene derivatives by the catalyst system Rh(CO) 2 acac/ultranox626/CO/H 2 /CH 2 Cl 2 led chemoselectively to 5,6-dihydronaphthalene derivatives 5a–d in good yields. The addition of a catalytic amount of H 3 PO 4 enhanced in situ the cyclization process via the nucleophilic attack on the carbonyl group of the linear aldehyde, and finally the elimination of alcohol. The type of substitution on phenyl group of the allylbenzene is of great importance in enhancing the cyclization process.