Abstract: Based on the color-fading reaction between amido black 10B (AB) and cationic surfactant in a weakly acidic medium of acetate buffer,sensitive spectrophotometric methods for the determination of CTMAB and CPB were proposed.The ion-association reaction between AB and CTMAB was carried out in an acetate buffer medium of pH 2.20,and the reaction between AB and CPB in an acetate medium of pH 3.60.Beer′s law was obeyed in the range of 0-2.09×10-5mol·L-1 (for CTMAB) and in the range of 0-2.17×10-5mol·L-1 (for CPB),with apparent molar absorptivity of 1.93×104L·mol-1·cm-1 and 2.16×104L·mol-1·cm-1 respectively.The detection limit for CTMAB was found to be 1.31×10-6mol·L-1,and for CPB was found to be 7.64×10-7mol·L-1.The proposed methods have been applied to the determination of cationic surfactants in water samples,giving values for RSD′s less than 1.5%,and recoveries in the range of 98.9%-101.0%.A study on the mechanism of the ion-association reaction between AB and cationic surfactant was also made by the density functional theory of quantum chemistry,the results obtained were in conformity with the experimental facts.