Mg-based Mg-Ti binary alloys have been synthesized by mechanical alloying of Mg and Ti powder blends. It was found that mechanical alloying of Mg and Ti results in a nanocrystalline Mg-Ti alloy and an extended solubility of Ti in Mg, due to the favorable size factor and the isomorphous structure of Mg and Ti. In the case of Mg-20at.% Ti, about 12.5% Ti is dissolved in the Mg lattice when the mechanical alloying process reaches a stable state. The rest ( about 7.5 at.%) remains as fine particles in the size of 50 - 150 nm in diameter. Dissolution of 12.5 at.% Ti in the Mg lattice causes a contraction of the unit cell volume from 0.0464 to 0.0442 nm(3) and a decrease of the c/a ratio from 1.624 to 1.612 of the hexagonal structure. The supersaturated solid solution Mg-Ti alloy decomposes upon thermal annealing at temperatures above 200 degreesC. Hydrogenation enhances the decomposition process at lower temperatures. (C) 2003 Kluwer Academic Publishers.