We have developed a new photon upconversion (UC) system utilizing a new amphiphilic sensitizer la that comprises a hydrophilic ruthenium complex and a lipophilic bisanthracene appendage. At concentrations higher than 5 mu M in toluene, the sensitizer la formed a reverse micellar assembly which facilitated the triplet sensitization of 9,10-diphenylanthracene (DPA) more efficiently than homogeneously dispersed solutions to enhance the UC efficiency up to 38.2%. The Stern-Volmer analyses revealed the stepwise triplet-triplet energy transfers (TTET): (1) intramicellar energy transfer from the ruthenium core to the bisanthracene surface and (2) diffusion-dependent energy transfer from the surface to DPA. On these bases, it can be assumed that the reverse micellar assemblies accelerate the former TTET process to enhance the UC efficiency.