In order to obtain efficient organic light-emitting diodes (OLED) it is necessary to funnel both singlet and triplet excitons generated by electroexcitation to the emitting dye molecule. Energy transfer plays thereby a decisive role. We have studied mechanisms and kinetics of the energy transfer process involving singlet and triplet excitations in a donor-acceptor system suitable for use as active layer in OLEDs. Fluorescent and phosphorescent tris(phenylquinoxaline) served as donor and a red emitting platinum porphyrin as acceptor molecule. The results of our investigations show that efficient energy transfer from the singlet and triplet excited state of the donor to the acceptor molecules takes place by combined long-range dipole-dipole interaction and diffusion. Due to the very different oscillator strengths of radiative transitions from the singlet and triplet excited state to the singlet ground state the rate of energy transfer of triplet excitons is several orders of magnitude slower than that of singlet excitons. Since, however, the lifetime of both states differ by the same order of magnitude the transfer efficiency is comparable.