Abstract: At least two fluorophores (e.g. fluorescein-biotin and tetramethylrhodamine-streptavidin conjugate) for use in fluorescence correlation spectroscopy, characterised in that the fluorophores have substantially the same excitation wavelength (a) and different emission wavelengths (b and d). The fluorophores are used for simultaneous excitation with a single laser wavelength in fluorescence cross-correlation spectroscopy (FCCS). The central wavelength (c) of the dichroic mirror used for the separation of the emission signal lies between the emission wavelengths of the fluorophores.