The rhodium catalyzed hydroformylation of P-isophorone (1a) should afford, as only oxo-product, the aldehyde 4-formyl-3,5,5-trimethylcyclohexan-1-one (2a), an important intermediate for the preparation of delta-Damascone, a floral woody fragrance used for soaps, shampoos, foam baths, etc. Surprisingly, under the reaction conditions adopted by us, we obtained two isomeric aldehydes, namely the expected 2a, formed in a small amount, and the (3,3-dimethyl-5-oxo-cyclohexyl)acetaldehyde (5a), that resulted to be the preponderant oxo-product. The chemoselectivity of the reaction was strongly affected by the substrate isomerization to a-isophorone (3a) and by the extensive formation of the corresponding saturated ketone 4a, so lowering the efficiency of the whole process. However, by perfoming the hydroformylation on a P-isophorone derivative without the possibility of conjugation of the two double bonds, namely the ketal 7,9,9-trimethyl-1,4-dioxa-spiro[4.5]dec-7-ene (1b), the chemoselectivity, of the reaction strongly increased and the amount of the hydrogenation product was rather low. In any case the predominant oxo-product was the acetaldehyde derivative 5b, while the aldehyde 2b, deriving from the attack of the formyl group on the less substituted carbon atom of the olefinic double bond, was produced in a very small amount. (c) 2005 Elsevier B.V. All rights reserved.