Abstract: Coadsorption in electrochemistry is a complex phenomenon involving several interactions of two or more than two species with the electrode surface. It is dif- ficult to investigate coadsorption with conventional electrochemical techniques that inevitably measure the sum of all the processes at the electrode/solution interfaces. In this article surface enhanced Raman spectroscopy (SERS)~[1-3] has been used to characterize the nature of the coadsorption process as the SER signals of these coadsorbed species can be detected simultaneously~[4]. By analysing relations of the SER spectra and the electro-chemical parameters, it is possible to classify coadsorption as two aspects: parallel coadsorption and induced coadsorption. Moreover, the latter can be further divided into two groups: induced chemical coadsorption and induced physical coadsorption.