A technique developed in our laboratory, which combines electroless plating and osmosis, was used for synthesis of thin dense Pd-composite membranes. Based on a physical model of the deposition process, the mechanism of osmosis influence on properties of I'd film is suggested and verified experimentally. It is shown that, (i) use of osmosis allows one to prepare thinner fully dense palladium films in shorter period of time, (ii) the metal penetrates deeper into porous support during electroless plating with osmosis, as compared with conventional plating, enhancing interaction between film and substrate and thus improving mechanical and thermal properties; (iii) films of finer and more uniform microstructure are synthesized by the use of osmosis. As a result, hydrogen permeation flux through the Pd-Vycor glass composite membrane increases approximately 2-fold as concentration of the sucrose osmotic solution employed during plating increases from 0 to 9 M. These features make osmosis a powerful tool to synthesize Pd-composite membranes with high hydrogen permeability and attractive thermal and mechanical properties for a wide variety of applications.