A new method for measuring triplet quantum yields is described and demonstrated. In this self-calibrated technique, one compares intensities of prompt fluorescence and delayed triplet-triplet annihilation fluorescence from the solution-phase sample of interest. This approach differs from other methods in that it requires no reference compounds, energy-transfer partners, or absolute measurements of triplet state energy, sample concentration, optical density, excitation energy, or emission intensity. Instead, the triplet quantum yield is determined from the emission time profile, using data from a single sample. Results from this method agree with published values to within ca; 0.02, even in the unfavorable case of a weakly fluorescent sample.