The kinetics and mechanisms of molybdate adsorption/desorption at the gamma-Al2O3/water interface were studied by using the pressure-jump apparatus with conductivity detection at 298 K. A double relaxation was observed due to the adsorption/desorption process. Adsorption data and triple-layer model (TLM) simulation results suggest the formation of both mono- and bidentate inner-sphere complexes (SMoO4- and S2MoO4) at the gamma-Al2O3 surface. The intrinsic equilibrium constants (K-eq(int)) of the complexes were 106.5 M-2 and 10(16) M-4, respectively. Based on the relaxation theory and combined results of TLM simulation, a two-step process is proposed. The first step (k(1), k(-1)) is the formation of an ion-pair complex through the electrostatic attraction between the reacting surface sites and MoO42- and H+. The second step (k(2), k(-2)) involves a ligand exchange process, whereby one water molecule is replaced by one adsorbed MoO42- from the surface. The values of adsorption and desorption rate constants in the MoO42-/ gamma-Al2O3 system were determined to be k(1)(int) = 5.23 x 10(6) M-2 s(-1) k(-1)(int) = 2.41 x 10 s(-1), k(-2)(int) = 1.74 s(-1), and k(-2)(int) = 3.26 x 10(-1) s(-1) The intrinsic equilibrium constant from kinetic measurements (K-kin(int)) was 10(6) M-2, which was similar to the intrinsic equilibrium constant (K-eq(int)), 10(6.5) M-2, from the equilibrium studies.