We show from time-resolved fluorescence intensity and depolarization experiments that the fluorescence of the unique tryptophan W119 of PYP is quenched by energy transfer to the 4-hydroxycinnamoyl chromophore. Whereas the intensity decay has a time constant of 0.18 ns in P, the decay in the absence of the cofactor (apo-PYP) has a single exponential lifetime of 4.8 ns. This difference in lifetime with and without acceptor can be explained quantitatively on the basis of energy transfer and the high-resolution X-ray structure of P, which allows an accurate calculation of the kappa2 factor. Fluorescence depolarization experiments with donor and acceptor indicate that both are immobilized so that kappa2 is constant on the fluorescence time scale. Using background illumination from an LED emitting at 470 nm, we measured the time-resolved fluorescence in a photostationary mixture of P and the intermediates I2 and I2'. The composition of the photostationary mixture depends on pH and changes from mainly I2 at low pH to predominantly I2' at high pH. The I2/I2' equilibrium is pH-dependent with a pKa of approximately 6.3. In I2 the lifetime increases to approximately 0.82 ns. This is not due to a change in distance or to the increase in spectral overlap but is primarily a consequence of a large decrease in kappa2. Kappa2 was calculated from the available X-ray structures and decreases from approximately 2.7 in P to 0.27 in I2. This change in kappa2 is caused by the isomerization of the acceptor, which leads to a reorientation of its transition dipole moment. We have here a rare case of the kappa2 factor dominating the change in energy transfer. The fluorescence decay in the light is pH-dependent. From an SVD analysis of the light/dark difference intensity decay at a number of pH values, we identify three species with associated lifetimes: P (0.18 ns), I2 (0.82 ns), and X (0.04 ns). On the basis of the pH dependence of the amplitudes associated with I2 and X, with a pKa of approximately 6.3, we assign the third species to the signaling state I2'. The absorption spectra of the 0.82 and 0.04 ns species were calculated from the pH dependence of their fluorescence amplitudes and of the photostationary light/dark difference absorption spectra. The lambda(max) values of these spectra (372 and 352 nm) identify the 0.82 and 0.04 ns components with I2 and I2', respectively, and validate the fluorescence data analysis. The mutant E46Q allows a further test of the energy transfer explanation, since lowering the pH in the dark leads to a bleached state with an increased spectral overlap but without the isomerization-induced decrease in kappa2. The measured lifetime of 0.04 ns is in excellent agreement with predictions based on energy transfer and the X-ray structure.