We have employed laser-induced liquid bead ion desorption mass spectroscopy (LILBID MS) to study the solution behavior of Pseudornonas aeruginosa azurin as well as two mutants and corresponding Re-labeled derivatives containing a Re(CO)(3)(4,7-dimethyl-1,10-phenanthroline)(+) chromophore appended to a surface histidine. LILBID spectra show broad oligomer distributions whose particular patterns depend on the solution composition (pure H2O, 20-30 mM NaCl, 20 and 50 mM NaPi or NH4Pi at pH = 7). The distribution maximum shifts to. smaller,Oligomers upon decreasing the azurin concentration and increasing the Puffer concentration. Oligomerization. IS less extensive for native azurin than its mutants. The oligomerization propensities of unlabeled and Re-labeled proteins are generally. comparable, and only Re 126 shows some preference for the dimer that persists even in highly diluted solutions. Peak shifts to higher masses and broadening in 20-50 mM NaP confirm strong azuring association with buffer ions and solvation. We have found that LILBID MS reveals the solution behavior of weakly bound nonspecific protein oligomers, clearly distinguishing individual components of the oligomer distribution. Independently, average data on oligomerization and the dependence on solution composition were obtained by time-resolved anisotropy of the Re-label photoluminescence that confirmed relatively long rotation correlation times, 6-30 ns, depending on Re-azuring and solution composition. Labeling proteins with Re-chromophores that have long-lived phosphorescence extends the time scale-of anisotropy measurements to hundreds of nanoseconds, thereby opening the way for investigations of large oligomers with long otation times.