Tetracationic water-soluble porphyrin ( H2P4+) has been immobilized by pi-pi stacking interactions onto the skeleton of carbon nanohorns (CNH), without disrupting their pi electronic network. The stable aqueous solution of the CNH- H2P4+ nanoensemble was examined by both electron microscopy and spectroscopic techniques. The efficient fluorescence quenching of the H2P4+ moiety in the CNH-H2P4+ nanoensemble was probed by steady-state as well as time-resolved fluorescence emission spectroscopy, suggesting charge separation from the photoexcited H2P4+ to CNH. In the presence of methyl viologen dication (MV2+) and a hole trap, accumulation of the reduced species of methyl viologen was observed by the photoillumination of CNH-H2P4+, suggesting that the electron migration from the initially formed charge- separated state takes place. Transient absorption spectroscopy gave further insights on the transient species such as the charge-separated state (CNH center dot-)-(H2P4+)(center dot+), which was consumed in the presence of MV2+ and hole shifter, leaving the reduced methyl viologen.