Abstract: Electrospray ionization-tandem mass spectrometry (ESI-MS/MS) was applied to the detection of 4,4′-diamino diphenyl oxide reduced from various azo dyes in textiles and leathers, without chromatographic separation. In the MS/MS analysis, ESI was used as ion source, and modes of SIM and SRM with positive ion scanning were taken for qualitative and quantitative analysis of 4,4′-diamino diphenyl oxide. The ［M+H］ (m/z 201) was taken as the mother ion, and the fragment ions ［M+H-NH3］ (m/z 184) and ［M+H-C6H7N］ (m/z 108) were taken as daughter ions. Limit of detection (19S/N) found for the method was 70 ng·L-1. Precision of the method was tested with sample introduction of 2 μL of 1 ng·L-1 4,4′-diaminodiphenyl-oxide solution for 9 determination, value of RSD found was 1.7%.