Monte Carlo and molecular dynamics simulations of interlayer molecular structure in the one-, two-, and (hypothetical) three-layer hydrates of K-montmorillonite were performed concurrently in order to elucidate counterion speciation and water structure in the electrical double layer of this clay mineral. Calculated layer spacings, interlayer water potential energies, and counterion mobilities were in agreement with available experimental data. In the one-layer hydrate, both outer-sphere and inner-sphere surface complexes of K+ were observed, the latter always near sites of tetrahedral charge substitution, with the counterion species exchanging readily on the simulation time scale (up to 200 ps). In the two-and three-layer hydrates, the surface complexes persisted, but an incipient diffuse layer of counterions also was observed, with all three types of surface species engaging in sluggish exchange. Water molecules in the one-layer hydrate resided at the interlayer midplane, whereas ill the two-layer hydrate they lay in two planes between outer-and inner-sphere K+ surface complexes, as veil as at the midplane. Hydrogen bonds in the one-layer hydrate were longer and more bent than in bulk liquid water. For all three hydrates, water and cation interlayer mobilities remained below those observed in bulk solution, principally because of the restricted geometry and the retarding effect of clay layer surface charge. Most of our results can be understood in terms of the weak solvation of the counterions by water molecules, which permits significant competition between K+ and water protons for negatively charged sites in the clay mineral.