Abstract: Aiming to the cognition of the mechanism of the reaction between the molecules of methylene blue (MB) and the molecules of large-molecular compounds,the interaction of MB with sodium dodecyl sulfate (SDS) was studied spectrophotometrically.The effects of the presence of ethanol,sodium chloride,hydroxypropyl-β-cyclodextrin and Triton X-100 on the reaction were also tested.As shown by the experimental results,a complex compound was formed between MB and SDS leading to change of color,and the color-change reaction was affected obviously by the addition of ethanol.It was concluded that the complex-compound of MB-SDS was formed on the basis of electrostatic action,and the directional aggregation of MB molecules bound on the orderly aggregates of SDS was the cause the color change action.