The steady-state and time-resolved fluorescence and 1H NMR spectra of a stilbenesulphonic acid salt, commonly used as an optical brightening agent, was studied as a function of concentration in aqueous solution. The aggregates, formed at higher concentrations, were shown to be predominantly dimers with the central stilbene moieties of the molecules stacked plane-parallel. The equilibrium constant for the dimer formation at 25°C was obtained from both NMR and fluorescence measurements.