The adsorption behavior of a CO adlayer at at Au core-Pt shell (Au@Pt) nanoparticle electrode as a function of CO coverage, partial pressure, and electrolyte composition is examined by electrochemical in situ surface-enhanced Raman spectroscopy (SERS) combined with a thin layer flow cell, which allows time-resolved spectro-electrochemical measurements upon sudden exchange of the electrolyte under potential control. We find (i) a clear decrease (increase) in the Pt-COL (C-O-L) peak frequency and all increase of both hands' intensities With COad Coverage; (ii) a slight increase in the Pt-COL and decrease in the C-O-L Stretching frequencies and an increase in both intensities of the COad-saturated layer when switching from CO-saturated to CO-free Solution; (iii) a clear decrease (up to 50%) of intensities of all of the Pt-CO and C-O bands and the SERS background during the Solution switch from H2SO4 or NaOH to Na2SO4; (iv) a slightly higher C-O-L (lower Pt-COL) peak frequency in H2SO4 than that in Na2SO4 and NaOH. The contributions of different factors such as SERS enhancement, Pt-CO binding strength, CO molecular orientation, and dynamic dipole-dipole Coupling interactions among the COad molecules to the SERS spectral behavior are discussed on the basis of the complementary information of both Pt-CO and C-O stretching vibrations. ; The Chinese Academy of Science ; National Natural Science Foundation of China (NSFC) [20773116]; State Key Laboratory of Physical Chemistry of Solid Surfaces of Xiamen University [200706]; Ministry of Science and Technology (MOST) of China [2010CB923302]