Persistent nonphotochemical hole burning and delta-FLN spectra obtained at 4.5 K are reported for monomeric chlorophyll (Chl) a/b light-harvesting complexes of photosystem II (LHC II) of green plants. The hole burned spectra of monomeric LHC II appear to be similar to those obtained before for trimeric LHC II (Pieper et al. J. Phys. Chem. R 1999, 103, 2412). They are composed of three main features: (i) a homogeneously broadened zero-phonon hole coincident with the burn wavelength, (ii) an intense, broad hole in the vicinity of similar to 680 nm as a result of efficient excitation energy transfer to a low-energy trap state, and (iii) a satellite hole at similar to 649 nm which is correlated with the low-energy 680 nm hole. Zero-phonon hole action spectroscopy reveals that the low-energy absorption band is located at 679.6 nm and possesses a width of similar to 110 cm(-1) which is predominantly due to inhomogeneous broadening at 4.5 K. The electron-phonon coupling of the above-mentioned low-energy state(s) is weak with a Huang-Rhys factor S in the order of 0.6 and a peak phonon frequency (omega(m)) of similar to 22 cm(-1) within a broad and strongly asymmetric one-phonon profile. The resulting Stokes shift 2S omega(m) of similar to 26.4 cm(-1) readily explains the position of the fluorescence origin band at 680.8 nm. Thus, we Conclude that the 679.6 nm state(s) is (are) the fluorescent state(s) of monomeric LHC II at 4.5 K. The absorption intensity of the lowest Q(y) state is shown to roughly correspond to that of one Out of the eight Chl a molecules bound in the monomeric subunit. In addition, the satellite hole structure produced by hole burning within the 679.6 nm state is weak with only one shallow satellite hole observed in the Chl b spectral range at 648.8 nm. These results suggest that the 679.6 nm state is widely localized on a Chl a molecule, which may belong to a Chl a/b heterodimer. These characteristics are different from those expected for Chl a612, which has been associated with the fluorescent state at room temperature. Alternatively, the 679.6 nm state may be assigned to Chl a604, which is located in a cluster with several Chl b molecules resulting in a relatively weak excitonic coupling.