There has been a surge of interest in the surface-enhanced Raman scattering (SERS) of p-aminothiophenol (PATP), since its SERS spectra are dependent on the measurement conditions. However, there is a dispute over the origin of the so-called b2 modes in SERS spectrum of PATP recently. Some researchers propose that these bands come from selective chemical enhancement, while others conclude that these bands are due to the ag modes of p,p'-dimercaptoazobenzene (DMAB) produced from PATP by surface photoreaction. To solve this problem, we have studied the SERS spectra of PATP on Au and Ag nanoparticles by in situ measurement under various conditions. The results proved that the b2 modes are not due to PATP but due to the ag modes of DMAB. The key of the method is to ensure the SERS spectra taken from the same point in reduplicative measurements. The result showed that the stable SERS spectrum of PATP was essentially from DMAB. The reversibility of the PATP SERS spectra in previous studies is due to the variety of the measurement points, which is in nature of different PATP conversions to DMAB under different conditions. Copyright © 2013 John Wiley & Sons, Ltd.