Homogeneous dispersion of microemulsion containing palladium nanoparticles in scCO(2) is, for the first time, observed via sapphire window reactor and these particles show an unusual reluctance for double bond hydrogenation of citral aldehyde at hydrophobic end rather than hydrophilic end (high regioselectivity) owing to the unique micelle environment in supercritical carbon dioxide that guide a head-on attack of the molecule.