In this paper, the influence of electrolyte concentration on the thickness of an adsorbed polyelectrolyte layer has been investigated. High molecular weight cationic polyacrylamides (M(w) similar to 5 x 10(6)) have been adsorbed on a negatively charged silica surface. The adsorbed amounts and the thickness of the adsorbed layer have been determined by ellipsometry. The amount of polyelectrolyte adsorbed at low ionic strength is to a first approximation the amount needed to give charge compensation of the surface. When the electrolyte concentration is increased up to 500 mM NaCl, there is a fourfold increase in the thickness of the adsorbed layer. During the first cycle in electrolyte concentration there is some hysteresis and also a slight desorption of polyelectrolyte. After the first cycle, the changes accompanying changes in electrolyte concentration are reversible, By making a rapid change in electrolyte concentration in the measuring cell, we have attempted to estimate the time for polyelectrolyte reconformation perpendicular to the surface. For these high molecular weight polymers the time constant for reconformation is of the order of 20-30 s.