Abstract: A new substraction spectrophotometric method for the simultaneous determination of cobalt and palladium with 4-(5-nitro-2-pyridylzao)1,3-diaminobenzene（5-NO2-PADAB） has been developed. The method is based on the chromatogenic differences in the different acidity conditions between the complexes of the two kinds of metallic ions with 5-NO2-PADAB. At the room temperature Co(Ⅱ) reacts with 5-NO2-PADAB and forms a stable complex at pH 6.2～8.0 and further acidification makes the maximum absorption wavelength of the complex shift from 518 nm to 580 nm with remarkable sensitivity improvement. While Pd(Ⅱ) reacts with 5-NO2-PADAB quantitatively in strong acid medium and forms a complex with the maximum absorption wavelength at 586 nm. At 586 nm the absorption of the complexes of Pd(Ⅱ) and Co(Ⅱ) shows a good addictive property with molar absorptivity of εCo=1.28×105 L·mol-1·cm-1 and εPd=1.04×105 L·mol-1·cm-1respectively. Beer′s law is obeyed in the concentration ranges of 0~0.48 mg/L for Co and 0~1.2 mg/L for Pd. The method has been applied to the simultaneous determination of trace Co and Pd in ores. And the results are in agreement with those from AAS with precision (n＝6）of 1.6%RSD for Co and 1.0%RSD for Pd respectively.