A model aq. 25-50% BuOH. (contg. 2.7% Na dodecylsulfate (SDS)) 1-hexene hydroformylation system was used to study the formation of microemulsions and the activity of a hydrophilic hydrogencarbonyltri[sodium tri(m-sulfophenyl)phosphine]rhodium catalyst. SDS, 2.7%, reduced the interfacial tension of H(2)O1-hexene (or n-hexane) from 31 only to 4 mN/; the system was emulsifiable. With (43:57%) BuOH-H2O and SDS > 17%, 150 g 1-hexene could be solubilized per 100 g water phase; 1-heptanal required 25% of SDS. In 50 g (43:57) BuOH-H2O, 0.1 mol 1-hexene was hydroformylated in 97% (2 MPa, 100degreesC, Rh 500 ppm) with (1:1) H-2-CO to yield 188 mmols/h aldehydes. The products were sepd. by adding BuOH-H2O to the system.