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Summary form only given. As is known, in collisionless case for the electrode boundary sheath formation it is necessary that positive ions at the sheath edge have a velocity V/sub 1//spl ges/V/sub s/=/spl radic/kT/sub e//M where V/sub s/ is the ion sound speed, T/sub e/ is the electron temperature, M is the ion mass. Godyak (1982) has modified the Bohm's criterion for a collisional case neglecting the friction force acting on electrons and estimated the electric field at the sheath edge as E/sub 1/=kT/sub k///spl lambda//sub D/ where /spl lambda//sub 1/ is the electron Debye radius at the sheath edge. Riemann (1991) has shown this value to be too high and suggested to use in a collisional case the following expression E/sub 1/=kT/sub e//(e/spl lambda//sub D//sup 2/5//spl lambda//sub 1//sup 3/5/) where /spl lambda//sub 1/ is the ion mean-free path. Valenti has determined the Bohm criterion for a collision plasma. However, Riemann et al. have derived from Valentini's results the nonmonotonous behaviour of plasma parameters in the presheath. In this paper the equation for the ion velocity V/sub 1/ at the plasma-sheath interface is obtained.