Photochemical hole-burning spectroscopy was used to study the excited-state electronic structure of the 4-hydroxycinnamyl chromophore in photoactive yellow protein (PYP). This system is known to undergo a trans-to-cis isomerization process on a femtosecond-to-picosecond time scale, similar to membrane-bound rhodopsins, and is characterized by a broad featureless absorbance at 446 nm. Resolved vibronic structure was observed for the hole-burned spectra obtained when PYP in phosphate buffer at pH 7 was frozen at low temperature and irradiated with narrow bandwidth laser light at 431 nm. The approximate homogeneous width of 752 cm⁻¹ could be calculated from the deconvolution of the hole-burned spectra leading to an estimated dephasing time of ∼14 fs for the PYP excited-state structure. The resolved vibronic structure also enabled us to obtain an estimated change in the C=C stretching frequency, from 1663 cm⁻¹ in the ground state to ∼1429 cm⁻¹ upon photoexcitation. The results obtained allowed us to speculate about the excited-state structure of PYP. We discuss the data for PYP in relation to the excited-state model proposed for the photosynthetic membrane protein bacteriorhodopsin, and use it to explain the primary event in the function of photoactive biological protein systems. Photoexcitation was also carried out at 475 nm. The vibronic structure obtained was quite different both in terms of the frequencies and Franck–Condon envelope. The origin of this spectrum was tentatively assigned.