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Measurement has been made of the critical transfer distances for five donor-acceptor systems-fluorene, carbazole, dibenzofuran, dibenzothiophene, and 9-fluorenol as donors and 9-phenylanthracene as the acceptor. The good agreement, between the experimentally measured and theoretically calculated critical transfer distances for the first three systems, indicates the applicability of Forster's dipole mechanism of resonance energy transfer. The measured and calculated critical transfer distances for the last two systems also agree, but the low values obtained make the applicability of Forster's dipole-dipole mechanism questionable. The fluorescence quantum efficiencies and the critical transfer distances show similar trends as the molecular structure is changed. The corrected fluorescence spectra and fluorescence quantum efficiencies of the five donor molecules have also been measured.