A hyperbranched polymer containing high concentrations of viologen units was employed to sensitively detect serotonin. The formation of CT complex between hyperbranched viologen polymer, HB-1 and serotonin was investigated by UV-visible absorption and fluorescence spectroscopy as a function of serotonin concentration in the absence and presence of L-tryptophan as a possible disturbing agent. Serotonin was sensitively and selectively detected down to 2 mu M in aqueous solutions even in the coexistence of L-tryptophan.