Coumarin 151, when complexed 1:1 by a water molecule in a supersonic jet, develops two sets of resonances, based on features displaced by -45 cm(-1) (species A) and -690 cm(-1) (species B) from the uncomplexed molecule. The vertical ionization thresholds of these species differ by a much greater amount (0.5 eV) than the bound states, confirming that the attachment sites are functionally different. When two water molecules are present, there appears to be a single cluster type, having an origin shifted from the bare chromophore by -1214 cm(-1). In this case, fluorescence excitation spectra recorded simultaneously at two emission wavelengths show that the 2:1 species exhibit a Stokes shift for excitation energies greater than or equal to 60 cm(-1). Also, above this energy barrier, the ionization energy shifts downward by >1000 cm(-1). The bare molecule also undergoes vibronically induced nonradiative relaxation, but this occurs at higher energies (i.e., >450 cm(-1)) and results in a large increase in the effective ionization threshold. The fluorescence decay time of the 2:1 water aggregate, excited into the zero-point level of S-1, was found to be twice that of the ban molecule (6.7 ns vs 3.5 ns). Above the relaxation barrier, the decay time of the initial state rapidly decreased below the instrument resolution of 35 ps. On the other hand, the fluorescence decay time of the product state increased to approximate to 9.5 ns and remained constant up to >400 cm(-1) excess vibrational energy.