This study investigates the removal mechanism of arsenate, i.e., As(V), by coprecipitation with aluminum hydroxide in solution at pH 5. Both the filtrates and precipitates were analyzed. The removal mechanisms of As(V) were investigated by constructing sorption isotherms and by analyses of X-ray diffraction (XRD) patterns, zeta potentials, and Fourier-transform infrared (FT-IR) spectra. The sorption isotherms showed Brunauer-Emmett-Teller (BET) behavior. The sorption densities showed a rapid uniform increase when the initial As/Al solution molar ratio was more than 1.5. This suggests two sorption mechanisms of As(V), namely surface complexation and three-dimensional uptake, which were dependent on the initial As/Al molar ratio. It is important to note that the As(V) and Al(III) concentrations were unable to reach the thermodynamic solubility product of aluminum arsenate in the bulk solution. The three-dimensional uptake was therefore estimated to be not bulk precipitation but surface precipitation of aluminum arsenate. Surface precipitation, generation of a new surface phase, is known to be derived from surface complexes. XRD, zeta potential, and FT-IR analyses of the precipitates for each As/Al molar ratio confirmed that As(V) was predominantly removed by the adsorption of the As(V) oxyanions to the aluminum hydroxide surface when the initial As/Al molar ratio was less than 1.5. At an initial As/Al molar ratio greater than 1.5 at pH 5, As(V) was removed mostly by surface precipitation, evidently by poorly crystalline aluminum arsenate on the surface. This sorption as surface precipitation is a phenomenon specific to the coprecipitation process.