Abstract: In an acetate buffer solution of pH 4.32,the binary complex formed by lanthanum(Ⅲ) and methyl blue reacted with kanamycin (KANA) or neomycin (NEO) to form blue ion association complexes,having positive absorption maxima at 682 nm for both KANA and NEO,with molar absorptivity of 1.71×104L·mol-1·cm-1 (for KANA) and 1.21×104L·mol-1·cm-1 (for NEO),and negative absorption maxima at 620 nm (for KANA) and 616 nm (for NEO),with molar absorptivity of 9.57×104L·mol-1·cm-1 (for KANA) and 8.68×104L·mol-1·cm-1 (for NEO). When the absorption measurement was made by DW-spectrophotometry,the values of molar absorptivity acheved to 1.10×105L·mol-1·cm-1 (for KANA) and 1.00×105L·mol-1·cm-1. In the analysis of a kanamycin injection sample and a neomycin ear drops sample,the results obtained were in consistency with the labelled values,and results of recovery tests were in the range of 99.8%-100.5%.