In this study, batch adsorption experiment coupled with potentiometric titration and spectroscopy techniques were applied to investigate the co-adsorption mechanisms of Cd(II) and As(V) to a soil bacterium Delftia sp. The results showed that carboxyl, phosphoryl, amino and hydroxyl groups abound in bacterial surfaces, with dissociation constant (pK_a) of 4.57, 7.04, 8.22 and 10.45, respectively; Batch results showed that the adsorption of Cd in binary system was larger than that in single system at pH<5, whereas no significant difference was found at pH>5; For As(V), the presence of Cd inhibited as adsorption over the whole pH ranges (2-9). XPS results demonstrated that carboxyl and amine groups participated in the complexation of heavy metals on bacterial surfaces, and Cd and As may compete with each other on the adsorption sites. This study confirmed that soil pH is an important factor affecting the co-adsorption of Cd and As on microbial surface.