The voltammetric behavior of methylene blue (MB, or MB þ in solution) at a silver electrode has been studied. MB þ can exhibit a cathodic peak at about À 0.2 V (vs. SCE), while the electrogenerated LMB (leucomethylene blue) exhibits an anodic peak at almost the same potential in neutral phosphate buffer. In the presence of some surfactants such as cetyltrimethylammonium bromide (CTAB), cetylpyridinium bromide (CPB) and gemini surfactant butyl-a,wbis(dimethylcetylammonium bromide) (C 16 ÀC 4 ÀC 16 ), the peaks, specially the anodic peak, become higher and shift to more positive potentials. This results from the associated adsorption or surface complexation of LMB/MB þ with adsorbed surfactants. Surfactants with same alkyl chain (i.e., À(CH 2 ) 15 CH 3 ) but different molecular structure, show different influence due to their different adsorbability and hydrophobicity. C 16 ÀC 4 ÀC 16 , bearing two hydrophobic alkyl chains, makes the MB peaks occur at more positive potentials, while CPB makes the peaks grow more rapidly. Dihexadecyldimethylammonium bromide (DCAB), showing weaker adsorption under this condition, has little influence on the peak height of MB. The influence of other factors is discussed as well.