To aq. 25-50% BuOH contg. Na dodecyl sulfate (SDS, > 6 g) or Na dodecylbenzene sulfonate (ABS, > 60 g/100 g aq. phase), 1-dodecene or 1-tetradecene was added until turbid (e.g. 100 g aq. 45% BuOH contg. 40 g SDS solubilized 90 g 1-dodecene) and hydroformylated (0.1 mol, 2 MPa, 100 degrees C, 1 hour, Rh 450 ppm, H-2 + CO = 1:1; (aq. 25% BuOH + 35 g ABS) or (aq. 50% BuOH + 20 g SDS) mixt., 50 g) with (2,4-pentanedionato)carbonyttri(m-sulfophenyl)phosphinerhodium as catalyst to yield 1-tri- or 1-pentadecanal in 91 or 97.5% yield, resp. At suitable BuOH and SDS concns., the post-reaction mixt. stratified spontaneously and gave the org. phase contg. the product aldehyde. Solubilization of reactant + product binaries in aq. 25% BuOH contg. 3% of tri(m-sultophenyl)phosphine trisodium salt was studied in relation to SDS and ABS concn.