Linear long chain dialdehydes, a class of valuable intermediates for a wide variety of commercially important products and of interesting cross-linking agents for protein and polysaccharides, can be conveniently prepared by hydroformylation of alpha,omega -diolefins and protected aldehydes having a terminal olefinic double bond. Thus, 1,5-hexadiene and 1,6-heptadiene were converted in a single oxo-process step into the corresponding linear dialdehydes in very high yields (80-98%), using RhH(CO)(PPh3)(3)/Xantphos (1:3) as the catalytic system at 40-60 degreesC and 20 atm (CO: H-2 = 1). If more stable monoprotected dialdehydes are needed, they can be obtained in a > 90% yield by hydroformylation of ethylene acetals of omega -vinylaldehydes, like 2-(9-decenyl)- and 2-(10-undecenyl)-1,3-dioxolane, catalysed by the above Xantphosrhodium complex under analogous reaction conditions. This catalytic system showed to be superior to platinum complexes such as PtHCI(PPh3)(3)/(SnCl2H2O)-H-., which require more drastic reaction conditions (100atm and 100 degreesC)to ensure yields of linear protected aldehydes ranging from 65 to 86%.