High catalytic activity (TurnOver Frequencies up to 700 h-1) was achieved using new triphenylphosphine-functionalized core cross-linked micelles, TPP@CCM, as nanoreactors and [Rh(CO)2(acac)] as catalyst precursor for the aqueous biphasic hydroformylation of 1-octene. While the hydrophobic core of the polymer offers an adequate environment for the catalytic reaction, its hydrophilic shell confines the catalyst into the aqueous phase and prevents extensive leaching towards the substrate/product phase. Selectivity is better than that of the homogeneous reaction catalyzed by the Rh/PPh3 system, with minor isomerization and linear to branched aldehyde ratios (l/b) between 3 and 6. Various operating parameters, such as catalyst/ligand concentration, temperature and P/Rh ratio, were varied and proved that these nanoreactors do not suffer from significant mass transfer limitations. The effect of the nanoreactor hydrophobic core size and degree of functionalization on activity and on l/b regioselectivity has also been investigated. The possible causes of catalyst leaching are discussed.