The time-dependent nature of the complexes of IR806 dye and other similar near-infrared cyanine dyes in human serum albumin (HSA) have been studied by employing absorption, emission and time-correlated single photon counting techniques. The complex formation of IR806 with HSA modifies the native structure of IR806, resulting in time-dependent changes in its optical properties. The modification process of the dye and its complex formation with HSA is very slow and takes about 90 min. The process of the formation of the new complex is irreversible and totally controlled by the initial complex of IR806 and HSA. As far as we know, this new property of cyanine dyes has not been reported before. These properties are very important for near-infrared fluorescence imaging.