An optical organic semiconductor microcavity showing the strong coupling regime was prepared. The dye 1,1 ' -diethyl-2,2 ' -cyanine (PIC) forming J-aggregates was used as optically active material in a planar lambda /2-microcavity configuration. Stable thin layers of homogeneously distributed aggregates without any matrix were prepared by spin-coating of a specific salt of the dye [(PIC+)(2)B10H102-]. Strong exciton-photon coupling was demonstrated by the anticrossing of exciton and photon modes. A Rabi-splitting of more than 50 meV was observed in transmission measurements. The asymmetric normal mode splitting could be explained by the asymmetric absorption spectrum of the J-aggregate.