In pH 4.5 Britton-Robinson (BR) buffer solution, erythrosin (ET)can react with diphenhydramine (DP) to form a 1:1 ion-association complex, which not only results in the change of the absorption spectra, but also results in the great enhancement of resonance Rayleigh scattering (RRS) and the quenching of fluorescence. Furthermore, a new RRS spectrum will appear, and the maximum RRS wavelength was located at about 580 nm.In this work, the spectral characteristics of the absorption, fluorescence and RRS, the optimum conditions of the reaction and the properties of an analytical chemistry were investigated. A sensitive, simple and new method for the determination of DP by using erythrosin as a probe has been developed. The detection limits for DP were 0.0020 μg/mL for RRS method, 0.088 μg/mL for absorption method and 0.094 μg/mL for fluorophotometry. There was a linear relationship between the absorbance, RRS and fluorescence intensities and the drug concentration in the range of 0.0067-2.0, 0.29-6.4 and 0.31-3.2 μg/mL, respectively. The effects of the interaction of diphenhydramine and erythrosin on the absorption, fluorescence and resonance Rayleigh scattering spectra were discussed. In light polarization experiment, the polarization of RRS at maximum wavelength was measured to be P = 0.9779, and it revealed that the RRS spectrum of DP-ET complex consists mostly of resonance scattering and few resonance fluorescence. In this study, enthalpy of formation and mean polarizability were calculated by AM1 quantum chemistry method. In addition, the reaction mechanism and the reasons for the enhancement of scattering spectra and the energy transfer between absorption, fluorescence and RRS were discussed.resonance Rayleigh scattering, spectrophotometry, fluorescence quenching method, diphenhydramine, erythrosin Diphenhydramine (DP) is an antihistamine. It blocks the effects of the naturally occurring chemical histamine in the body. It is used to treat all kinds of symptoms of allergies and suppress coughs and to treat mild forms of Parkinson's disease. However, DP and its metabolites have a series of side effects, such as dizziness, headache, extreme sleepiness, even eclampsia, exanimation, cardiac inhibition, respiratory paralysis. In combination with other drugs, it may induce hypertension and arrhythmia [1,2] . In order to get better therapeutic effect and less side effects, it is very necessary for pharmaceutical analysis and clinical medicine to determine diphenhydramine quantitatively. At present, only high concentration of DP can be determined by titration method [3] because of its low sensitivity and difficulty for judging the end point. Many analytical techniques including spectrophotometry (SP) [4,5] , spectrofluorometry (F) [6,7] , highperformance liquid chromatography (HPLC) [8,9] , gas chromatography (GC) [10] , liquid chromatography-mass spectrometry (LC-MS) [11] , capillary electrophoresis